Using the Pythagorean Theorem to find a missing leg
We can use the Pythagorean theorem to find a missing leg of a triangle, but only if we know the length measure of the hypotenuse and the other one of the legs.
Find x

5^{2}=3^{2}+x^{2}
x^{2}=5^{2}3^{2}
x^{2}=259
x^{2}=16
x=4

In a right triangle you know the hypotenuse is 20 units long and one leg is 16 units long.
Calculate
the other leg: