Three identical 6 kg cubes are place in a horizontal frictionless surface in contact with one another. The cubes are lined up from the left to the right and a 36 N force is applied to the left side of the left cube causing all three cubes to accelerate to the right. if the cubes are each subject to a frictional force of 6 N, what is the magnitude of the force exerted on the right cube with the middle cube in this case?