Force exerted on one object by another

[anti404]

Homework Statement

http://lon-capa.bsu.edu/res/brookscole/serway/College_Physics_7ed/serwaypictures/sf4_27.jpg

m1=1.2kg
m2=2.3kg
m3=3.1kg
a[of the system] determined to be = 6.36m/s^2
T[between the cord of 1-->3] determined to be =22.27N

Determine the force exerted by the number 1 block on the number 2 block.

Homework Equations

Newton's Second Law, I imagine; sigma F = mass*acceleration

The Attempt at a Solution

if the two were connected via a cable or something, I'd get how to do it; as stands, I'm not really sure. we've only went over this briefly, so I'm quite stumped.
any help would be appreciated, however!

 

[w3390]

A clearer explanation of the setup of all blocks, ropes, and pulleys would be helpful.

 

[anti404]

I put up an image showing it, but it isn't displaying, now.
I'll put it up again.
http://img163.imageshack.us/img163/9191/sf427.jpg