First try

src/main/java/formula/Square.java

 package formula;
 
 public class Square implements Formula {
+    private Formula formula;
+
     public Square(Formula formula) {
+        this.formula = formula;
+    }
+
+    @Override
+    public String asString() {
+        return "(" + formula.asString() + "^2)";
+    }
 
+    @Override
+    public double asValue() {
+        return Math.pow(formula.asValue(), 2);
     }
 }