Question

A company estimates that it will sell N(x) hair dryers after spending $x thousand on advertising as given by: N(x)=-0.25x^4+13x^3-180x^2+10,000 for 15<=x<=24 When is the rate of change of sales increasing, decreasing?(Hint: Find N'(x) which is the rate of change, then find N"(x) to check when N'(x) increasing or decreasing)

Answer 1

I wanted to know why n'(x) isn't -1x^3+39x^2-360x?