Start at the origin. Call the length from the origin going directly along the x-axis to the end of the rectangle "x". Thus if you went the other way along the x-axis you would go another distance of "x", and the total length of the base is "2x". Now since the upper corners are on the graph of , that's a give away that your height can now be given by . So, area of a triangle is , which gives you an area function of . Now differentiate and set the derivative equal to zero to find the critical point(s). And once you get you critical x-value, plug that into your area function and you should have your answer.