How do I find the velocity of this

A 4800-kg truck traveling with a velocity of +4m/s collides head-on with a 1200-kg car traveling with a velocity of -12 m/s. the truck and care entangle and move together after the collision. Determine the velocity of the truck and car immediately after the collision.

I know that the answer is .8 m/s. but how do I find it? Do I use the inelastic collision formula?

yes you use the inelastic collision formula


m1v1 + m2v2 = (m1 + m2) vf

where vf is the final velocity you're trying to solve for


4800 kg (4m/s) + 1200 kg (-12 m/s ) = (4800 + 1200) vf

Find momentums of each.
(4800 x 4) = 19,200kg/m/sec.
(1200 x 12) = 14,400kg/m/sec.
Clearly the truck has more momentum than the car, so when they collide, they will move in the direction the truck was moving (+).
(19,200 - 14,400) = 4,800kg/m/sec. of momentum left.
(4800 + 1200) = total mass of 6,000kg.
(4,800/6,000) = + 0.8m/sec.