what's the answer to my question?

Answer:
17
Step-by-step explanation:
The Pythagorean Theorem is L^2 + L^2 = H^2:
We can just substitute 15 and 8 as our L's:
So, 15^2 + 8^2 = H^2.
225 + 64 = 289.
Square root 289 = 17.
So 17 feet is the length of the hypotenuse.