For this problem we can take advantage of newtons third law by finding the force of the big block on the little block we will know the force of the little block on the big block.

So the acceleration of the little block is 2 m/s^2 so the force on the little block by the big block is F=ma= (2kg)(2m/s^2) = 4N. Now this the this is the newton third law pair of the force on the big block by the little block. So the force on the big block due to the little block is the negitave of the force we found so the final awnser is