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