Ok, in this type of excercise you have to imagine a triangle. This one is a right triangle defined by tree points that I will label A, B and C. Those points are: the base of the tower (A), the top of the tower (B) and the point on the ground where the zip line ends (C). The angle we are being told is between the ground (segment AC) and the zip line (segment BC) therefore is the angle
If we rewritte this equation with our values: