If
(x + 1/x) = 7
what is
(x^3 + 1/x^3) ?
This being a puzzle rather than just a maths problem, there is of course a way to solve it without having to go through the messy business of working out what x is equal to. You might even want to let (x + 1/x) = A and then express (x^3 + 1/x^3) purely in terms of A.