Related Rates... I think

Suppose i have a closed box, the volume of this box is fixed, say, 60in^3.

The material for the top and the bottom of the box cost twice as much as the material for the sides. How would I go about minimizing the costs. I know it has something to do with taking the derivative = 0, but other than that, i'm stumped... Any help here would be much appreciated.

hints:
1. make a function height->cost
2. assume width and length are equal (unless you have to prove that they must be to maximize bottom/top area)
3. the second derivative has to be positive as well (otherways its a local maximum)