Given the instantaneous position is
x=t3−6t2+20t+15...(1)
The velocity of the particle is given by
v==dtdx3t2−12t+20...(2)
And, the acceleration of the particle is given by
a==dtdv6t−12...(3)
When a=0, it implies
6t−12=0⇒t=2s
At t=2s, the velocity can be calculated as follows:
v==3(2)2−12(2)+208ms−1