In Fig. 5-42, elevator cabs A and B are connected by a short cable and can be pulled upward or lowered by the cable above cab A. Cab A has mass 1700 kg and cab B has mass 1200 kg. A 12.0 kg box of sand lies on the floor of cab A. The tension in the cable connecting the cabs is 1.86 x 10^4 N. What is the magnitude of the normal force on the box from the floor? I don't even know where to start, please help?