using:
a^2 + b^2 = c^2
let: a = x,b = 16,c = 20.
hence:
x^2 + 16^2 = 20^2
x^2 = 20^2 - 16^2
x^2 = 400 - 256
x^2 = 144
taking the square root of both sides:
gives, x = 12.
