The particles in the figure below (m1 = 1.8 kg and m2 = 3.5 kg) undergo an elastic collision in one dimension. Their velocities before the collision are v1i = 12 m/s and v2i = ?7.5 m/s. Find the velocities of the two particles after the collision. (Indicate the direction with the sign of your answer.) v1f = . m/s v2f = . m/s