This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
On line 10 you're successfully calling the coolMethod method of Zoo, via an instance of Zoo, which is z. If you want to call the coolMethod method on your instance of Moo, first you need to define it in Moo somehow. Yes, inheriting from Zoo would be one way. Just adding the method to Moo would be another - which is better depends on why you're doing it.
Once you've done that, you can call it on the current instance on line 11 by using this.coolMethod(), or just coolMethod().