# Need help with an inelastic collision problem

A car weighing 900 kg is waiting at a stop sign. A car weighing 1200 kg hits the oter car. The cars move .76 m after the collision. The coefficient of friction between the sliding tires and the pavement is .92. I need to find the speed of the car right before the collision.
I dont know what to use. Conservation of momentum doesnt work because I need at least one of the velocities.

You got to find the final momentum. The idea is that the cars stick together after collision. So the final momentum is $(m_1 + m_2)v_{final}$. You got to find $v_{final}$.
You can do that. The cars are decelerated by a constant force of friction. This force is $F = .92(m_1 + m_2)g$. From $F$ and $d = .76m$ you can calculate $v_{final}$.