Just need C and D

The position of an object moving along a line at any time t 2 0 is given by the function s(t) = +3 - 9t2 + 18, where s is measured in feet and t is measured in seconds. A) Find the velocity of the particle at t = 4. B) Find the acceleration of the particle at t = 4. C) Was the speed of the particle increasing or decreasing at t = 4 ? Justify your answer. D) For t > 0, find the acceleration when the particle changes direction