You can use the sin function to solve this.
Think of a triangle with the conveyor being the longest side (hypotenuse). The height (the vertical side) is H=17m. The angle of elevation is a=36.2. We are looking for length of the hypotenuse, L.
The sin is the ratio of height to hypotenuse: sin(36.2) = H/L = 17/L
Now solve for L:
L = 17/sin(36.2) = 17m/0.59 = 28.78m
The conveyor needs to be approx. 28.8m long to get close to the max. efficient angle.
