Let f be the function describing the line. Since it passes through (4,5), where (b>0) is the slope of the line. The shape formed in the first quadrant by this line and the axes is a triangle whose height is the y-intercept of the line and whose width is its x-intercept. You can compute the intercepts yourself to verify that they are and . The area of the triangle is therefore . Can you take it from here by setting the derivative to be zero? You should obtain , I think.