Do you know how to find the area of a rectangle? Suppose we call the length of the fence parallel to the river "x" and the length of fence perpendicular to the river "y". What is the formula for the area of the rectangle. What amount of fencing will he need for the two sides of the rectangle parallel to the river? How much for the one side parallel to the river? You can use the fact that he has 500 feet of fencing available to write "x" in terms of "y" (or vice-versa) so that the are depends only on one variable.