[SOLVED] optimization problem (cost function)

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.

$\displaystyle

V=20=2x^2y

$

$\displaystyle

Surface Area = 2x^2+6xy

$

Cost Function:

$\displaystyle

15*(2x^2) + \frac{7*60}{x}

$

$\displaystyle

30x^2 + \frac{420}{x}

$