Here are a few more videos to watch for extra help:

http://www.khanacademy.org/math/calculus/differential-calculus/v/optimization-with-calculus-1

http://www.khanacademy.org/math/calculus/differential-calculus/v/optimization-with-calculus-2

http://www.khanacademy.org/math/calculus/differential-calculus/v/optimization-with-calculus-3

http://www.khanacademy.org/math/calculus/differential-calculus/v/optimization--example-4