Two blocks m1

and m2 sit on a horizontal frictionless surface and are in contact with one

another. A force F is applied to the box on the left (m1). The force that m2 exerts on m1 is known to be F12 .What is the applied force F in terms of m1, m2, and F12?

I know the force body diagrams but i having trouble actually coming up with the relationship?