Both cars are canceling out the energy of each other. The energy that is left over because one car has a bigger kinetic energy is the energy that is going to move BOTH the cars.
First Car : 0.5 * 2000kg * 3m/s^2 = 10.24 KJ
Second Car : 0.5 * 2000kg * 2m/s^2 = 4 KJ
Leftover Energy : 10.24 KJ - 4 KJ = 6.24 KJ
Movement of BOTH cars :
6.24 = 4000kg * v^2
v = √(6.24 / 4000) =0.04 m/s = 4 cm/s
