The velocity of the train is constant at 21 m/s. So it's position as a function of time is x_train = x_0 + vt = 60 m + 21m/s x t
The car is accelerating, so it's position is determined by:
x_car = v_i x t + 1/2 at^2 = 35m/s x t + 1/2 x 3 m/s^2 x t^2
Set these two equations equal, and solve for t. IYou can then put that value for t into an equation for the velocity of the car:
v_car = v_i + at
