A farmer has a large field that is x feet in length. He wants to fence in a rectangular section in the middle of the field, leaving a length of 100 feet of open field beyond each end of the fenced rectangle. He also wants the width of the fenced-in space to
be 100 feet less than its length, as shown in the diagram. Find the equation in standard form for the area of the fenced-in section in terms of the length of the field.