Arailroad car of 2000 kg coasting at 3.0 m/s overtakes and locks with an identical car coasting on the same track in the same direction at 2.0 m/s. what is the speed of the cars after they lock together?

The key is in "they lock together". This tells you this is a complete inelastic collision and therefore you can use the formula:
m1*v1+m2*v2=v(m1+m2)
Plug in numbers:
2000*3+2000*2=v(2000+2000)
so v=2 m/s, obviously in the same direction of motion in which they were both moving.

Your answer is c. explanation: plasma  is the fourth state of matter, derived from the gaseous state of matter.  plasma  can contain free  electrons  and/or  ions. while  plasma is never neutral, it can either be positively or negatively charged.  gases, however, are electrically neutral.