I can find the cost function, but I don't know how to find minimum cost. Which should = $329.34

A rectangular storage container with an open top is to have a volume of 20 m^3. The length of its base is twice the width. Material for the base costs $15 per square meter.Material for the sides costs $7 per square meter.

1. Find the cost function for the container.

2. Find the cost of materials for the cheapest such container.

Cost Function: