Courses on Khan Academy are always 100% free. Start practicing—and saving your progress—now: https://www.khanacademy.org/math/multivariable-calculus/applications-of-multivariable-derivatives/optimizing-multivariable-functions-videos/v/multivariable-maxima-and-minima A description of maxima and minima of multivariable functions, what they look like, and a little bit about how to find them.