Force equation for horizontal component of a two-block system

[iMAGICIALoTV]

Homework Statement

Homework Equations

I don't know if I set the equation for horizontal component of the bigger block correctly:

$\huge{\Sigma F_{x_1} = F_a - (f_k + N_2) = ma}$

If this is true then it seems that there is not enough given information to find $F_a$. Are there some other way?

The Attempt at a Solution

Find the normal force and friction, then substitute some variables, I guess? I got 6.877 N, but I know it's wrong...

 

[iMAGICIALoTV]

Somebody? Help me, please?

 

[howie8594]

Find the force of gravity on the .4kg object. That'll give you the required upward friction. Then find the normal force that the 1.8kg object must exert on the .4kg object to get that required friction.