The formula used to find the hypotenuse in this problem is called the Pythagorean Theorum. The formula is as follows: a^2 + b^2 = c^2. Or, you can say that the hypotenuse is equal to: c = the square root of a squared and b squared added together. This might sound confusing in the start, but here's all you have to do. a = 9, and b = 12. 9 squared is 81. 12 squared is 144. 144 + 81 = 225. The square root of 225 is 15, because 15 x 15 = 225. So 15 is your answer.
Ooh, look! This triangle is a Pythagorean triple! Without even needing to use a formula, I can tell that you are correct and the answer is A. 15.
It's based off of a 3-4-5 right triangle, because 9-12-15 simplifies to that if you divide every term by 3. It's basically just a bigger version of a 3-4-5 triangle! Isn't math cool:)
