Finding the acceleration of an object

1. The problem statement, all variables and given/known data
In the picture:
There is a force (F) pushing the car to the right
The car’s mass is M
There are two other objects with masses m1 and m2 that are connected by a cord with no mass.
There is no friction
In terms of m1, m2 and M find the acceleration needed to keep m1 and m2 from falling down.

2. Relevant equations

This is the second Newton’s law.
The only equation I use is


3. The attempt at a solution

I have tried doing this by placing the 3 m’s (m1, m2 and M) in their respective Cartesian planes to get the forces of each one, but the part I don’t quite understand is which forces I have to put in the formula…
I understand that I need to add the masses:
and I understand that Fnet consists of the F force that is affecting the car, but what other?
Do I need to use the tension of the cord that holds m1 and m2 together? or all I do is:
So the acceleration would be
F/(M+m1+m2)=a ?

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