To answer this, you are trying to find C^2, which is the hypotenuse.
You can use Pythagorean Theorem to solve this problem, which is A^2+B^2=C^2.
When substituting, the equation becomes 9^2+12^2=C^2.
Which is then 81+144=C^2, and then 225=C^2.

The answer is 15.