Your moving company runs out of rope and hand trucks, so you are forced to push two

Question:

Your moving company runs out of rope and hand trucks, so you are forced to push two crates along the floor as shown in Figure P3.39. The crates are moving at constant velocity, their masses are m₁- 45 kg and m₂- 22 kg, and the coefficients of kinetic friction between both crates and the floor are 0.35. Find the normal force between the two crates.

Figure P3.39