How do I calculate the tension of string? I have already calculated the acceleration of the 2 masses which is 3m/s. But I can't seem to arrive at the answer which is 9.00N. Sorry if this was a straightforward question.
As you correctly found acceleration of the centre of mass, and hence of both the blocks (as both must be moving with same acceleration to keep the string taught) is $3m/s^2$. Now draw a free body on 3kg mass, it is moving with $3m/s^2$ acceleration with only force acting on it i.e tension. So tension = 3*3=9N