Answer

a) Momentum before and after the collision = 0.45 kg x 0.60 m s^-1 - 0.30 kg x 0.42 m s^-1 (1) = 0.144 kg m s-1 (1).

Combined speed after collision = 0.144 kg m s^-1 / 0.75 kg = 0.19 m s^-1 (1)

b) Momentum of lighter vehicle before collision = (-) 0.45 kg x 0.60 m s^-1 = (-) 0.27 kg . s^-1 (1)

Speed = 0.27 kg m s^-1 / 0.30 kg = 0.90 m s^-1 (1)