Mathematics I'm working on a calculus assignment and need help understanding how to solve optimization problems using derivatives.

A rectangular garden must have an area of 600 square feet. The length is twice the width. How do I determine the dimensions that minimize the amount of fencing required?

Please provide:

* A step-by-step explanation

* The equations used Geometry Dash Lite 

* A brief explanation of why each step is necessary

I don't understand the process so I can solve similar problems on my own.