The hypotenuse is always opposite the right angle and it is always the longest side of the triangle. To find the length of leg a, substitute the known values into the Pythagorean Theorem. Solve for a^2. Side A and B does not matter when your trying to apply this to the Pythagorean theorem but side C must always be the hypotenuse. The hypotenuse is always the triangle's longest side. It is opposite the right angle.
