Edit: forgot that the function is cubic. Now to solve for g(x), you have to find the the inflection point. Inflection point is the point in which the second derivative of the cubic functions changes signs depending on the reference value of x. After that you have to find the a point of intersection of the two functions by setting them equal to each other. Now make the line go through the inflection point and the two other point of intersections. I assume that anyline within the inflection point and the point of intersection would create and equal amount of upper area to lower area. Not really sure though, but give it a try hehe