That MCAT will not know what hit it!

To solve this problem we will add up the total momentum before and after. Before the collision we have (4,500)(9) + - (4,000)(10) and after we have (8,500)(x). The 8,500 is both of the masses together because the two cars stick to each other in an inelastic collisions. Watch out for that negative sign! The negative sign indicates a direction.