Could someone explain step by step how to solve this, please?

Unit 2.4: Block A has a mass M and is being pushed by a hand with a force, FH. In front of block A is another object, block B, with a mass 2M. The two blocks move together. Derive an expression for the force that block A exerts on block B (FAB) as they move to the right with a constant acceleration (ax). Your nal expression should be in terms of M, 54, F AB, and other fundamental constants