First try

src/main/java/formula/Formula.java

 package formula;
 
 public interface Formula {
+    String asString();
+    double asValue();
 }
+

src/main/java/formula/Product.java

+package formula;
+
+public class Product implements Formula {
+    private Formula x;
+    private Formula y;
+
+    public Product(Formula x, Formula y) {
+        this.x = x;
+        this.y = y;
+    }
+
+    @Override
+    public String asString() {
+        return "(" + x.asString() + "*" + y.asString() + ")";
+    }
+
+    @Override
+    public double asValue() {
+        return x.asValue() * y.asValue();
+    }
+}

src/main/java/formula/Sum.java

 package formula;
 
 public class Sum implements Formula {
-    public Sum(Formula left, Formula right) {
+    private Formula x;
+    private Formula y;
+
+    public Sum(Formula x, Formula y) {
+        this.x = x;
+        this.y = y;
+    }
+
+    @Override
+    public String asString() {
+        return "(" + x.asString() + "+" + y.asString() + ")";
+    }
+
+    @Override
+    public double asValue() {
+        return x.asValue() + y.asValue();
     }
 }
+

src/main/java/formula/Variable.java

 package formula;
 
 public class Variable implements Formula {
-    public Variable(String variable, double i) {
+    private String name;
+    private double value;
+
+    public Variable(String name, double value) {
+        this.name = name;
+        this.value = value;
+    }
+
+    public void set(double value) {
+        this.value = value;
+    }
+
+    @Override
+    public String asString() {
+        return name;
+    }
+
+    @Override
+    public double asValue() {
+        return value;
     }
 }