I have this question: If the diesel engine coasts at 4.8 km/h into a freight car that is initially at rest, find the speed of the coupled cars. Now I'm looking at the work done, on the answers given but I'm not understanding where the 5.8 coming in to play?

Consider the phase change taking place in the picture. you could use all but one statement to describe what is happening here. that is: a) temperature is increasing. b) kinetic energy is increasing. c) the space between the particles remains the same. d) the attractive force between the particles is decreasing.