diff --git a/.rustlings-state.txt b/.rustlings-state.txt
index 64e0b71..fd19189 100644
--- a/.rustlings-state.txt
+++ b/.rustlings-state.txt
@@ -1,6 +1,6 @@
 DON'T EDIT THIS FILE!
 
-strings1
+modules1
 
 intro1
 intro2
@@ -38,4 +38,8 @@ structs3
 enums1
 enums2
 enums3
+strings1
+strings2
+strings3
+strings4
 quiz2
\ No newline at end of file
diff --git a/exercises/09_strings/strings2.rs b/exercises/09_strings/strings2.rs
index 93d9cb6..9e605a6 100644
--- a/exercises/09_strings/strings2.rs
+++ b/exercises/09_strings/strings2.rs
@@ -6,7 +6,7 @@ fn is_a_color_word(attempt: &str) -> bool {
 fn main() {
     let word = String::from("green"); // Don't change this line.
 
-    if is_a_color_word(word) {
+    if is_a_color_word(word.as_str()) {
         println!("That is a color word I know!");
     } else {
         println!("That is not a color word I know.");
diff --git a/exercises/09_strings/strings3.rs b/exercises/09_strings/strings3.rs
index f5e45b0..c293005 100644
--- a/exercises/09_strings/strings3.rs
+++ b/exercises/09_strings/strings3.rs
@@ -1,13 +1,26 @@
 fn trim_me(input: &str) -> &str {
-    // TODO: Remove whitespace from both ends of a string.
+    // TODO: Remove whitespace from both ends of a string
+    // can find the 1st not whitespace and last not whitespace and then return slice
+    let f = input
+        .find(|c: char| !c.is_whitespace())
+        .unwrap_or(input.len());
+    let r = input.rfind(|c: char| !c.is_whitespace()).unwrap_or(0);
+    if f < r {
+        &input[f..r + 1]
+    } else {
+        &input[..input.len() - f]
+    }
+    // input.trim()
 }
 
 fn compose_me(input: &str) -> String {
     // TODO: Add " world!" to the string! There are multiple ways to do this.
+    input.to_owned() + " world!"
 }
 
 fn replace_me(input: &str) -> String {
     // TODO: Replace "cars" in the string with "balloons".
+    input.to_owned().replace("cars", "balloons")
 }
 
 fn main() {
@@ -20,6 +33,7 @@ mod tests {
 
     #[test]
     fn trim_a_string() {
+        assert_eq!(trim_me("   "), "");
         assert_eq!(trim_me("Hello!     "), "Hello!");
         assert_eq!(trim_me("  What's up!"), "What's up!");
         assert_eq!(trim_me("   Hola!  "), "Hola!");
diff --git a/exercises/09_strings/strings4.rs b/exercises/09_strings/strings4.rs
index 4730726..ae7599e 100644
--- a/exercises/09_strings/strings4.rs
+++ b/exercises/09_strings/strings4.rs
@@ -1,5 +1,5 @@
 // Calls of this function should be replaced with calls of `string_slice` or `string`.
-fn placeholder() {}
+fn placeholder(_input: String) {}
 
 fn string_slice(arg: &str) {
     println!("{arg}");
@@ -13,7 +13,7 @@ fn string(arg: String) {
 // Your task is to replace `placeholder(…)` with either `string_slice(…)`
 // or `string(…)` depending on what you think each value is.
 fn main() {
-    placeholder("blue");
+    string_slice("blue");
 
     placeholder("red".to_string());
 
@@ -27,9 +27,9 @@ fn main() {
 
     // WARNING: This is byte indexing, not character indexing.
     // Character indexing can be done using `s.chars().nth(INDEX)`.
-    placeholder(&String::from("abc")[0..1]);
+    placeholder(String::from("abc")[0..1].to_string());
 
-    placeholder("  hello there ".trim());
+    placeholder("  hello there ".trim().to_string());
 
     placeholder("Happy Monday!".replace("Mon", "Tues"));
 
diff --git a/solutions/09_strings/strings3.rs b/solutions/09_strings/strings3.rs
index dcf2377..ee6b56a 100644
--- a/solutions/09_strings/strings3.rs
+++ b/solutions/09_strings/strings3.rs
@@ -1,4 +1,49 @@
-fn main() {
-    // DON'T EDIT THIS SOLUTION FILE!
-    // It will be automatically filled after you finish the exercise.
+fn trim_me(input: &str) -> &str {
+    input.trim()
+}
+
+fn compose_me(input: &str) -> String {
+    // The macro `format!` has the same syntax as `println!`, but it returns a
+    // string instead of printing it to the terminal.
+    // Equivalent to `input.to_string() + " world!"`
+    format!("{input} world!")
+}
+
+fn replace_me(input: &str) -> String {
+    input.replace("cars", "balloons")
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn trim_a_string() {
+        assert_eq!(trim_me("Hello!     "), "Hello!");
+        assert_eq!(trim_me("  What's up!"), "What's up!");
+        assert_eq!(trim_me("   Hola!  "), "Hola!");
+        assert_eq!(trim_me("Hi!"), "Hi!");
+    }
+
+    #[test]
+    fn compose_a_string() {
+        assert_eq!(compose_me("Hello"), "Hello world!");
+        assert_eq!(compose_me("Goodbye"), "Goodbye world!");
+    }
+
+    #[test]
+    fn replace_a_string() {
+        assert_eq!(
+            replace_me("I think cars are cool"),
+            "I think balloons are cool",
+        );
+        assert_eq!(
+            replace_me("I love to look at cars"),
+            "I love to look at balloons",
+        );
+    }
 }
diff --git a/solutions/09_strings/strings4.rs b/solutions/09_strings/strings4.rs
index dcf2377..3c69b97 100644
--- a/solutions/09_strings/strings4.rs
+++ b/solutions/09_strings/strings4.rs
@@ -1,4 +1,38 @@
-fn main() {
-    // DON'T EDIT THIS SOLUTION FILE!
-    // It will be automatically filled after you finish the exercise.
+fn string_slice(arg: &str) {
+    println!("{arg}");
+}
+
+fn string(arg: String) {
+    println!("{arg}");
+}
+
+fn main() {
+    string_slice("blue");
+
+    string("red".to_string());
+
+    string(String::from("hi"));
+
+    string("rust is fun!".to_owned());
+
+    // Here, both answers work.
+    // `.into()` converts a type into an expected type.
+    // If it is called where `String` is expected, it will convert `&str` to `String`.
+    string("nice weather".into());
+    // But if it is called where `&str` is expected, then `&str` is kept `&str` since no conversion is needed.
+    // If you remove the `#[allow(…)]` line, then Clippy will tell you to remove `.into()` below since it is a useless conversion.
+    #[allow(clippy::useless_conversion)]
+    string_slice("nice weather".into());
+
+    string(format!("Interpolation {}", "Station"));
+
+    // WARNING: This is byte indexing, not character indexing.
+    // Character indexing can be done using `s.chars().nth(INDEX)`.
+    string_slice(&String::from("abc")[0..1]);
+
+    string_slice("  hello there ".trim());
+
+    string("Happy Monday!".replace("Mon", "Tues"));
+
+    string("mY sHiFt KeY iS sTiCkY".to_lowercase());
 }