This is a basic problem based on Newton's Laws of Motion. Refer the below given pics for the detailed solution. The same problem can be solved by proceeding in slight different manner also i.e. by applying Newton's second law separately on both the blocks; thus getting two equations with two unknowns (acceleration and mutual force). One more thing to note here that while solving the problem the external force of 100N has been assumed to act on the 2kg block in Horizontal direction as it has not been clearly specified in the statement of the problem.