physics

At car (1140-kg)traveling at 76.04 m/s and truck (12600-kg)traveling 6.88 m/s have a head-on collision and then stick together. What is their final common velocity (m/s)? (assume the car is going in the positive direction)
What is the cars' change in momentum in the above question (kg m/s)?

When I calculate the velocity of the truck, I get 22.9 m/s which is correct.
The formula that I am using to find the common velocity is
pi= (velocity car)(mass car) - (velocity truck)(mass truck)

I then take the pi calculated and use it to find the final velocity

vf = pi/(mc+mt)

I get close to the right answer (-4.36) but should have been -4.42.

For the last question. I believe that I am using the wrong formula.
mc*(vf-vc)
The correct answer was -31100 but I get nowhere near this number.

Am I missing some key steps here?

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