The manager of the Footloose sandal company determines that t months after initiating and advertising campaign, S(t) hundred pairs of sandals will be sold, where

S(t)=3/t+2 - 12/(t+2)^2 + 5

(a) Find S'(t) and S"(t)

(b) At what time will sales be maximized? What is the maximum level of sales?

(c) The manager plans to terminate the advertising campaign when the sales rate is minimized. When does this occur? What are the sales level and sales rate at this time?

I got A) Which is:

s'(t)=-3(t-6)/(t+2)^3

s''(t)=6(x-10)/(x=2)^4

b) I found the root of derivative s' and got t=6, I plugged it into the original equation and got 5 1/24....I'm stuck, I don't know what to do next or even if I'm doing it right.