The total monthly revenue R(x) generated from sales of Calculus Videos is related to the amount x spent on advertising by Rm: 3x3 + 45x2 + 90x + 40 where x is in thousands of dollars. Calculate the point of diminishing returns for advertising costs. The point of diminishing returns occurs at x = :I . This means that if more than :I dollars is spent on advertising, then each additional dollar spent produces a return on investment than the previous dollar