How does friction affect the middle box in a double tension problem?

[greenir]

Im getting stuck on the fnet equation for the middle box. I don't really know where to start I can get the two fnet=ma equations for the boxes hanging and how do the boxes hanging on each side effect the system as a whole.
Thanks I know this is most likely an easy question !

 

[haruspex]

The first thing is to figure out which way friction will act on the middle box.
Post whatever you have.