The position of an object as a function of time is given as x = At3 + Bt2 + Ct + D. The constants are A = 2.10m/s3, B = 1m/s2, C = −4.10m/s, and

Question

D = 3m.(a) at what time(s) is the object at rest

Answer 1

at rest means that the velocity dx/dt = 0
so, you need to solve
3At^2 + 2Bt + C = 0
now plug in your numbers and work your magic.