The shape on the left is a square with side length of 12 cm
This means the height of the triangle is 12cm.
Knowing the height and the hypotenuse using the Pythagorean you can solve for the base.
20^2 = 12^2 + x^2
400 = 144 + x^2
Subtract 144 from both sides:
256 = x^2
Take the square root of both sides
X = sqrt(256)
X = 16
X = 16
