I wouldn't usually make a thread like this here but I am completely at a dead end here with this differentiation question. An area of hill farm alongside a straight stone wall is to be fenced against sheep, and 400m of fencing is available. The breadth of the enclosure is x m. What is the greatest rectangular area that can be enclosed? This is the one kind of differentiation question at my level which I can never do. If anyone could help it'd be appreciated. Also, if you would like to, discuss your hatred or love of maths.