Can you please help me understand this question, please and thank you!

3. A 900 kg car driving 20 m/s collides with a 1,100 kg truck going 15 m/s. The vehicles bounce off each other and the car moves back at 18 m/s. What's the velocity of the truck after the collision? Initial (before) Final (after) 20 m/s 15 m/s 18 m/s V = ? MA = 900 kg MB = 1, 100 kg elastic collision