A particle moves along a straight line as follows: it accelerates uniformly from 0 to 12m/s in the first 4s, then decelerates uniformly to −6m/s in the next 3s, and finally continues at this constant velocity of −6m/s for another 2s. Taking the positive direction as the initial direction of motion, determine the total displacement of the particle in the 9s.
(a) 18m
(b) 21m
(c) 3m
(d) 33m