A farmer has a plot of land. They have 80 feet of fencing. They wish to create a rectangular animal pen, using this fencing, and the back wall of the...

A farmer has a plot of land. They have 80 feet of fencing. They wish to create a rectangular animal pen, using this fencing, and the back wall of the plot of land, as shown in the diagram below:

What length must they make the animal pen to maximize the area of the animal pen?