A farmer has an 80m length of fencing. He wants to use it to form

three sides of a rectangular enclosure against an existing wall which

will be the fourth side.

a) Denote the length of one of the sides by x, what is the area of

the enclosure?

b) By use of differentiation or otherwise and the maximum area of

the enclosure.

I do not know how I can get the answer by using differentiation .. Help me please !