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 N(x) units of a product after spending $x thousand on advertising, as given by

N(x)= -0.25x⁴ + 23x³ - 540x² + 80,000           24<=x<=45

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.