The space in question is bounded by the parabola and the X axis. If
the rectangle has its base on the x-axis, then the upper corners are
touching the parabola. Also, note that if we solve for X and find Y,
then X * Y = ½ the rectangle (on the right side of the Y-axis. The
other half is on the left side of the Y-axis. Therefore, the total
area = 2X * Y.