Add new solution in variables ex. 4

solutions/variables.md

@@ -35,6 +35,17 @@ fn main() {
 
 4.
 
+```rust
+fn main() {
+    define_x();
+}
+
+fn define_x() {
+    let x = "hello";
+    println!("{}, world", x);
+}
+```
+
 ```rust
 fn main() {
     let x = define_x();
@@ -141,4 +152,4 @@ fn main() {
     // fill the blank to make the code work
     assert_eq!([x, y], [3, 2]);
 }
-```
+```