Force of three boxes

[strugglin-physics]

Three blocks of mass 12.5 kg each, on a frictionless horizontal surface are in contact with each other as shown in the figure below.



A force F = 91.4 N is applied to block 1 (mass m1). Determine the acceleration of the system (in terms of m1, m2, and m3).

I did 91.4 = 12.5a
91.4/12.5 = 7.312 but something is missing to that equation. how do I do it in terms of m1, m2, and m3)?

[Tide]

Since all three blocks are in contact you are attempting to accelerate all three of them - i.e. the combined mass.

[strugglin-physics]

That was an obvious answer that I should've gotten, thank you.

Could you give me a pointer on this one too, having to do with the same problem?
Calculate the force of contact that block m1 exerts on m2.

[Doc Al]

Calculate the force of contact that block m1 exerts on m2.
Apply Newton's 2nd law to block m1 alone.