Acceleration of a Car The distance s (in feet) covered by a car t seconds after starting is given by the following function. s = −t3 + 5t2 + 18t (0 ≤ t ≤ 6) Find a general expression for the car's acceleration (in ft/sec3) at any time t (0 ≤ t ≤ 6). s''(t) = At what time t (in sec) does the car begin to decelerate? (Round your answer to one decimal place.) t =