Define a method hypotenuse that calculates the length of the hypotenuse of a right triangle

That method would be an offshoot of the Pythagorean Theorem. Given that you have a right triangle, find the sum of the squares of the lengths of the two shorter sides, and then find the positive square root of this sum. That's the length of the hypotenuse.

If the shorter side lengths are a and b, then the length of the hypotenuse is sqrt (a^2 + b^2).