Math Problem Pythagorean theory?

So this is the question: The hypotenuse of a right triangle is 13cm. The sum of the shorter 2 sides is 17cm. Calculate the lengths of those two sides. So would I set it up as X(squared) + X(squared) = 13(squared)? Please help, I wanna understand this so I can do the rest of the problems. My teacher isn't good at explaining and maybe someone out there can help! thanks [: