point of diminishing returns. A company estimates that it will sell N(x)units of a product after spending

Question:

point of diminishing returns. A company estimates that it will sell N(x)units of a product after spending $x thousand on advertising, as given by
N(x)=-0.25x4 + 13x3 - 180x2 + 10,000 15 ≤ x ≤ 24
When is the rate of change of sales increasing and when is it decreasing? What is the point of diminishing returns and the maximum rate of change of sales? Graph N and N' on the same coordinate system.