Let

We can do it by substitution, but I wouldn't personally. However, it's good to start from there.

So I'm going to let

This means that so

Factoring the :

Applying for :

But as ,

So

Now, I let right at the start of my substitution. This means that , so

Giving us the required final answer of

Is this clear? Usually I'd ask you to do part of the problem yourself but I don't think there's really a good place to stop with this question.