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 ,
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.