The diagram for this problem is a pulley. On the left end Box 1 is sitting on a table and Box 2 is attached sitting on top of Box 1 and is attached to the string for the pulley. The right side has the string attached to Box 3 which is lifted off the table.
A massless rope passes over a massless, frictionless pulley. One end of the rope is connected to box 2 and the other end is connected to box 3. The masses of the three boxes are
Box 1: 5.4 kg
Box2: 3.7 kg
Box 3: 2.6 kg
Determine the magnitude of the normal force that the table exerts on box 1.
I drew free body diagrams to get me started but can't get much farther than that