Let the height of the box = h
Let the width/length of the box = x
We are given the volume and we are asked to find dimensions of the box. More specifically, the dimensions that will MINIMIZE the cost of building the box.
Plug the given volume into your volume equation and isolate your variable "h". Then substitute that into your area equation, so you will only have an equation in terms of x. Simplify. Take your derivative. To calculate the dimensions to minimize the cost, put your derivative = 0 and solve for x. Then plug your answer for x into the equation you used to substitute "h" and solve.
Then you can probably take it from here!