Yes, and so that gives you what equation?
Well, eventually. What I'm looking for is the next step in simplifying the equation in post # 21.
You see, in mathematics, it is not enough to get the right answer. You must know how you got the right answer. Some mathematicians, it is true, are so focused on the method of solving a problem that they either don't care about getting the answer at all, or they don't check to see if it's the right answer. The engineers get that one right. Overall, I'd say it is equally important to A. Get the right answer by B. Using a correct, valid, and applicable method.
Perhaps. But you haven't convinced me that you know why you had to take the cube root. Why not the fourth root? Why not square the number? Why not subtract 15?
If you don't know the why of your method, then that will not help you the slightest bit in tackling a new problem you haven't seen before. Math is not about applying known formulas to known problems. It's about thinking logically and imagining solutions to unknown problems.