When is the rate of change of sales increasing and when is

Question

Advertising point of diminishing returns. A company estimates that it will sell units of a product after spending $x thousand on advertising, as given by

N(x) = -0.25x⁴ + 11x³ - 108x² + 3,000

9<=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 on the same coordinate system.