Updated TODO in strings3.rs

exercises/09_strings/strings3.rs

@@ -1,13 +1,17 @@
 fn trim_me(input: &str) -> &str {
     // TODO: Remove whitespace from both ends of a string.
+    input.trim().to_string();
 }
 
 fn compose_me(input: &str) -> String {
     // TODO: Add " world!" to the string! There are multiple ways to do this.
+    let result = input.to_string() + " world!";
+    result
 }
 
 fn replace_me(input: &str) -> String {
     // TODO: Replace "cars" in the string with "balloons".
+    input.replace("world", "rustaceans")
 }
 
 fn main() {