I am not too sure which part you are having trouble with, so I will go through my own solution.

We have

(just substituting for and )

(Pythagorean identity)

But note that must be nonnegative (since is nonnegative when ). Similarly, So, we can solve for but we must throw out the negative root as it is not part of the original curve. So we get

(because )

and our final answer is