Question

what if the hypotenuse is 20 and one of the legs are 12 what is the length of the other leg?

Answer 1

x^2 + 12^2 = 20^2

x^2 = 400-144 = 256
x = √256 = 16

or
you might recognize that 20-16-12 is just a multiple of
the popular 5-4-3 right-angled triangle.