Weird Physics inelastic collision problem?

Question

A drunk driver strikes a parked car. During the collision the cars become entangled and slide to a stop together. The drunk driver's car has a total mass of 742 kg, and the parked car has a total mass of 776 kg. If the cars slide 18 m before coming to rest, how fast was the drunk driver going? The coefficient of sliding friction between the tires and the road is 0.59.

Err any ideas?

Answer 1

m = m1+m2
energy lost in slide = post-collision kinetic energy
Vf is post-collision velocity
18*0.59mg = mVf^2/2
Vf = sqrt(2*18*0.59g)
V1 = Vf*m/m1