Two crates, of mass 75kg and 110kg, are in contact and at rest on a horizontal surface

Question:

Two crates, of mass 75kg and 110kg, are in contact and at rest on a horizontal surface (Fig. 4-54). A 620-N force is exerted on the 75-kg crate. If the coefficient of kinetic friction is 0.15, calculate
(a) The acceleration of the system, and
(b) The force that each crate exerts on the other.
(c) Repeat with the crates reversed.