Arturo O. answered • 03/02/18

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a = d

^{2}s/dt^{2}ds/dt = -3t

^{2}+ 16t + 20a(t) = d

^{2}s/dt^{2}= -6t + 16At 8/3 sec,

a(8/3) = -6(8/3) + 16 = 0

a > 0 when t < 8.3 (positive acceleration)

a = 0 when t = 8/3 (instantaneously zero acceleration)

a < 0 when t > 8/3 (negative acceleration, i.e. deceleration)