Supposing the car collides in such a way that the car and the truck start moving together at the same velocity, then you can apply the law of conservation of linear momentum.
Calling the initial velocity of the car [tex] v_{0} [/tex] and the final velocity of car+truck [tex] v_{1} [/tex] :
[tex]m v_{0}=(m+2m) v_{1} [/tex].
Therefore [tex] v_{1} = \frac{1}{3} v_{0} [/tex].
