Question

hey can someone on here explain how i can find the tension in an object (a rope, in this case)?

basically box a (3kg) is accelerating +1m/s to the right (being pulled by a rope attached to a pulley system, there is no incline) while on the other side of the rope is box m (unknown mass). I know the normal force and the gravitational force cancel out, and that the tension is the same throughout the whole rope, but how would i calculate it? thanks