Using the Pythagorean Theorem to find the hypotenuse
We can use the Pythagorean theorem to find the hypotenuse of a right angled triangle, but only if we know the length measure of the two legs.
Find x

c^{2}=a^{2}+b^{2}
c^{2}=8^{2}+12^{2}
c^{2}=64+144
c^{2}=208

In a right triangle you know the legs are 12 and 9 units long.
Calculate
the hypotenuse length