If we know the angle at C is a right angle, what equation relates the length of the 3 sides?
a2 + b2 = c2
This is the Pythagorean theorem. Make sure that a and b are the legs and that c is the hypotenuse (the longest side).
There are three commonly seen sets of numbers (called Pythagorean triplets) that satisfy the Pythagorean theorem: {3, 4, 5}, {5, 12, 13} and {8, 15, 17} (Don't forget that multiples of these also work!)