Hi there smartartbug, happy new year!
I agree that after substitution
now
Now set and solve
Spoiler:
The problem is: You have to design a 500ft^3 square-based, open-top, rectangular holding tank. It is to be made from stainless steel plates. Find the dimensions for the base and height that will make the tank weigh as little as possible.
I know that you take 500=h*x^2 and find that h=500/(x^2). Then I think you're supposed to plug it into the SA equation and get SA=(x^2)+4x[500/(x^2)], but I'm not sure and I keep on getting impossible answers after that.