please help

Question 7 [6] The level of carbon monoxide emissions during peak traffic (6:00 to 9:00 in the morning) in Johannesburg is given by the function L = f(t) where L, in suitable units, is a function of t, the time in hours since 6:00 in the morning. Use a number line to determine when the level of emissions is decreasing given that f' (t) = kt?(t - 3)3(3t -4) with k > 0