If a car rolls gently off a vertical cliff, how long does it take to reach 90km/hr.

90 km/h : 3.6 = 25 m/s. If you know that on earth g = 9.81 m/s^2, then all you have to do is divide the speed by g. 25/9.81 = 2.548 seconds

At least, if by 'gently rolls off a vertical cliff' means that your starting velocity equals zero.