The answer would be 17.
This is because, the format for finding out the hypotenuse of a triangle is--
a^2 + b^2 = c^2
So, it would translate to--
15^2 + 8^2 = c^2
225 + 64 = 289
Because 289 = c^2, you need to take the square root of it.
The square root of 289 is 17.
Therefore, your answer is 17.