I have an optimization problem:
The base of an decorative rectangular feature with a volume of $V = 315$ ft$^3$ is to be constructed in a restaurant. The bottom is made of marble, the sides are made of glass, and the top is open. If marble costs five times as much (per unit area) as glass, find the dimensions of the feature that minimize the cost of the materials. (Assume that the length is greater than or equal to the width. Give your answers correct to at least three decimal places.)
I have createde a constraint equation that I am unsure of: $$lwh - (lw) = 315$$
The rest of the steps are a mystery to me.
I am asked to find the dimensions that would minimize cost for length, width and height.