At what time(s) is the rate of production increasing most rapidly?

46. The amount A(t) of widgets produced by a manufacturer is given by the function A(t) = 5000 + 36(t - 4) - 3(t - 4)3 where t is the number of hours of production since the start of the day at 7:00 am. At what time(s) is the rate of production increasing most rapidly