Consider the two block system in the image. One block ($mass = m$) is sitting on a table connected by a rope over a pulley to another block ($mass = 3m$) hanging in the air. There is a frictional force between the block resting on the table and the table equal to the weight of this block. In other words, $f=W$.
Which of the following is the correctly derived expression for the acceleration of the two block system?