We can eliminate 7 and 11 because the length has to be longer than 12 (called hypotenuse, it is the longest side of a triangle). And 225 is kind of ridiculous. We can also use the Pythagorean Theorem which applies to all right angle triangles to figure out the hypotenuse, so 9^2 (81) + 12^2 (144) = x^2. So then, 81 plus 144 is about 225. And the square root of 225 is 15, therefore the answer is 15. Hope this helps!
