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:)
The area and the width are known as 60 and 12 respectively. Rearranging that equation above, and plugging the values, one gets the value of the length of the rectangle as 5.
