Find the initial velocity of truck before collision

[tjae]

Question: A car of mass 1500kg collides head-on with a truck of mass 9000kg. After the collision, the wreckage is at rest. The speed of the car just before the collision was 30m/s. What was the speed of the truck just before the collision?

Relevant Equation(s):

m1v1+m2v2 = m1v1'+m2v2' (?)

My Attempt:

(1500kg)(30m/s)+(9000kg)( ___m/s) = (1500kg)(0m/s)+(9000kg)( 0m/s)
(1500kg)(30m/s)+(9000kg)( ___m/s) = 0
(45000 kg*m/s)+(9000kg)(___m/s) = 0

Problem: It's 1 in the morning and I'm not understanding what I'm doing wrong. Am I using the right equation? Do I need to set up west and east on the x-axis??

 

[Orodruin]

Why do you think you are doing something wrong? You will get a negative answer, but that only tells you the truck is moving in the opposite direction of the car.