math
posted by phil .
can someone help me with this question. I am completely stuck.
A box with a quare base and top has a volume of 20 meters cubed. The material for the base costs $7.00 per sq. meter, the material for the sides costs $4.00 per sq. meter, and the material for the top costs $2.00 per sq. meter. Express the cost of the materials used to build the box as a function of the width x of the base.

If the width of the box is x, and the volume is 29 = x^2 h, then the height h is 20/x^2.
Write a formula for the side area, the top area and the bottom area in terms of x only.
Finally, write another formula that adds up the costs of all sides.