Car pulls trailer of mass 500 kg . The friction acting on the car is 1200 n and that on the trailer is 400 N . At first the acc of the car  and trailer system is 2 m/s^2

What is the tension In coupling btw the car and the trailer for this acc? 

Hi Niha.  A great place to start would be a free body diagram of the trailer.  You have friction to the left (400N), and the tension to the right.  You also know it is accelerating at 2 m/s/s, so you could write Newton's 2nd Law, F=ma, as T-400N=500kg(a), and solve for T.