After the collision probably means that we are only interested in the motion after the cars have locked together. It's a little deceptive because it's worded like a conservation of momemtum problem. But it's really a free body diagram or Newton's second law problem.

After the collision, there is only one unbalanced force acting on the system: friction.

Newton's 2nd Law: ∑F=ma

Solving for a: a=∑F/m=f/m

(f = force of friction)

f=μF_{N}

F_{N}=mg

f=μmg

a=μmg/m=μg=0.85*9.8m/s^2=8.33m/s^2