This is just some pythagorean theorem, or a^2 + b^2 = c^2. We have a and b, which is 9 and 12, and we need to find c. To find c, we need to first square those, giving u 81 and 144. Now, we need to add them, giving us 225, meaning c^2 = 225. However, we need to find c on its own, so we have to find the square root of 225. That would be 15.
