The displacement of particle starting from rest is given by S=3t2 - t3 , where  t is in sec.The particle will come to rest again at  time of

displacement=S=3t2−t3to find velocity, differentiate above equation with respect to time Velocity V=dsdt⇒V=ddt3t2−t3V=6t−3t2 If V=00=6t−3t2⇒6t=3t2⇒t=63⇒t=2sec