Car A and B are passing each other in different lanes. Car A has an initial speed of 15m/s and is gaining speed at 1.5m/s^2. Car B has an initial speed of 20m/s and is gaining speed at 2.0m/s^2. Find when and where the cars will pass one another and their speeds as they are passing.

When -
I calculated this and found the time to be 4s. I'm sure of that answer.

I don't know how to calculate where and their speeds.

The time that is takes B to pass A depends upon how far behind it was at t=0. You did not give that information.

If you know position X vs time for both cars and solve for t at passing, using X1 = X2, then to get the velocity of each car use
V1 = 15 + (1.5 t
V2 = 20 + 2 t