I am having trouble setting up this problem in my physics HW.

I have tried many different things with the force with friction formula.

$\displaystyle F_fr=u_k*F_n$

I have tried and I don't know how to set up this problem

correctly.
The problem is

Three masses, M1= 3.3 kg, M2= 4.1 kg, and M3= 7.1 kg are made of the same material. They are on a horizontal surface

and connected by massless strings. The system accelerates

to the right at 2.43 m/sdue to an external force **F **pulling on M3.

Given that T1 is 17.44 N.

Calculate the coefficient of

kinetic friction μk? Calculate the magnitude of the force **F?**

Thanks