The first thing that would probably come to mind when solving this problem would be to plug in the numbers into the equations. Like this 12^2 + 16^2 = x^2 (X = to the number that you plug in from the answer bank.). This is correct thinking so you can try this:
144 + 256 = 400.
So your square has to be equal to 400.
And it most common knowledge that the square root of 400 is equal to 20.
So your answer is 20.
