So, in order to solve the problem, we must use the Pythagorean theorem, a^2 + b^2 = c^2. In this case, a=15 and b=8. So, when we plug these values into the equation and solve for c, we should get the correct diagonal.
15^2 + 8^2 = c^2
225 + 64 = c^2
289 = c^2
Square root(289) = square root(c^2)
17 = c
The diagonal should measure 17 ft. :)
