You can use the pythagorian theorem here: The corner on the top side is 90 degrees too. So that means (9+7)^2 = A^2 + B^2. The two triangles against each other are the same, but bigger. That means that the left side is 9/16 out of the whole and the right side is 7/16 out of the whole So back to the pythagorean theorem (9+7)^2 = 256. Multiply that number by one of the two numbers before. I will take 9/16, the left side. 256x9/16 = 144. Take the square root of that, 12, and then you have the length of that side. Then use pythagorean theorem again. C^2 = 9^2 + 12^2 C^2 = 225. C= 15 Seen as C stands for y, there is your answer