a^2 + b^2 = c^2
where a and b are the sides, and C is the hypotenuse (longest side)
basically
a + 13^2 = 16^2
so
a + 169 = 256
to find what A is, 256 - 169 = a = 87
but, because a is squared, you need the square root of 87 if it is a whole number, but it isn't-
Answer = [tex] \sqrt{87} [/tex]