Physics Qustion 1

Two cars are traveling one after another in a straight, flat road in a velocity of 90kmh-1. The car going in front suddenly slows down in a deceleration of 7.5ms-2. The car coming behind starts slowing down 0.5s later.  That car's deceleration is 6ms-2. To avoid hitting each another, what is the distance there should be between the two cars, when the car in front starts slowing down?