When the cаrs cоllide the relаtive pоsitiоn of B with respect to A is zero. So one wаy to find when they collide is to set your equation from problem 3 equal to zero and solve for t (You do not have to do that) Using your equation from problem 3 find the velocity of B with respect to A as a function of time Hint: you have relative position of B with respect A as a function of time, how can you turn it into a velocity as a function of time?