Optimization Problem. 10 points to best answer?

A man wants to bulid a box. He needs a square bottomed, closed box containing 252 cubic feet of space. The bottom costs $5 per square foot, the top $2 per square foot, and the sides $3 per square foot. Find the function for the cost, then use calc to find the dementions of the box that will minimize the cost, and state the minimum cost........... Please help me at least set up the function because I have no idea where to start. I tried 5x+12x+2x=252 but i dont think thats right. Please show your work if you do know how to answer this problem. Thank you