4.An object moves along a horizontal line in a way that its position is described by the function s(t)=\frac{1}{3} t^{3}-4 t^{2}+12 t-6,0 \leq t \leq 8 \text {, where } s \text { is in metres and } t \text { is in seconds. } (a) At what time(s) does the object stop? (b) At what time(s) does the object have an acceleration of zero? (c) Use your previous answers to determine during which time intervals the object is speeding up and slowing down.