Force needed to give both carts acceleration of 0.8 m/sec/sec
F = (3+2) 0.8 = 4 N
Force exerted on B by A is = 2 * 0.8 = 1.6 N
I suppose there is no force F (as in part (a) acting on A. There is only a force acting on B in the opposite direction.
force exerted by B on A is = mass of A * acceleration = 3 * 0.8 = 2.4 N
in the opposite direction
since accelerations are same , forces are different,. The reason is the difference in mass or INERTIA.
F = m a => Force is proportional mass. Or, higher force is needed to give the same acceleration to an object of higher mass.