Question

cooper's car runs out of the gas as it is going up a hill. The car rolls to a stop, then starts rolling backwards. As it rolls, its displacement, d(t) feet, from the bottom of the hill at t seconds since Cooper's car ran out of gas is given by d(t)= 99+30t-t^2
b) at what time did Cooper's car stop rolling up and start rolling back? how far was it from the bottom of the hill at his time?