Once again I am stuck on a question. Thanks as always.

Find all pairs of positive integers a and b such that...

a^2 + b^2 - 7 = ab

I've done this:

a^2 - ab - (7-b^2) = 0

But after that?

I've also found two solutions:a=2, b=3 and a=3 and b=2.

But i lack a solid algebraic proof.

Thanks!