Instead of putting the methods directly in the object, use prototype now to make the code complete.
Note that putting methods in Prototype is a lot more memory efficient than putting methods directly in the methods (as done in the last few exercises). If you're not planning to instantiate lots of objects, putting methods in Prototype won't be as needed. If you're planning to only instantiate once (or create a singleton), then you won't need to put the method in the Prototype.
As a lot of javascript codes would still follow this older convention, it's a good idea to get familiar with this concept of prototype and how to add things there.
Having what you learned above, now use prototype to make the code complete.