I’m assuming this is Reverse Pythagorean Theorem, so the equation would be c^2 - a^2 = b^2, so all you have to do it substitute the variables and solve. The length of the cable is variable c, the height of the tower is variable a, and the length of where is the cable is anchored and the base of the tower is variable b. That might not make any sense but feel free to ask any questions. :)
