Take it one step at a time.

Starting with:

and

This means we can rewrite this as:

Cubing this:

Take it one step at a time, starting with

= using the rule that

We can generalize this to have:

So do this to each term, remembering to cube the .

Once you get there, it just becomes a case of cancelling.

It looks like you're nearly finished with your simplifying. So no worries.