any help would be greatly appreciated
You'll get bobak's equation.
But since squaring both sides of an equation is not an equivalent operation you must prove if the solution is valid or not. Plug in the solution into the original equation and prove if the equation is true:
....... So y = 21 is a valid solution.
Cube both sides. You'll get:
Plug in this value into the original equation:
....... which is obviously true.
And therefore x = -6 is a solution of this equation.