Minimize cost of rectangular container with 10 m³ volume

[hks118]

Homework Statement

A rectangular storage container with an open top is to have a volume of 10 m^3. The length of this base is twice the width. Material for the base costs $10 per square meter. Material for the sides costs $6 per square meter. Find the cost of materials for the cheapest such container.

Homework Equations

See below

The Attempt at a Solution

I have my worked out (but wrong) solution here: http://img510.imageshack.us/img510/7963/calchw.jpg
For some reason I keep getting a negative answer. I really have no idea what I'm doing wrong.

 

[hotvette]

Your expression for y in terms of x isn't correct.