Physics education research, electronic materials, and the musings of Christopher Moore, Ph.D.

A.) A driver on a highway with an initial velocity of 30 m/s hits the breaks to avoid an accident. If the car can decelerate at 7 m/s2, then how much time will it take for the car to come to a stop (vf=0 m/s)?
B.) If it can only break 3 seconds before it rear ends the other car, then what will be its speed of impact?

I tried A. but I'm not sure if I did it right. I did -7=-30/t to find t and got about 4.29s? But I'm totally confused on how to solve B..
Thanks in advance!