So I'm just supposed to calculate the tension in the wire (see attachment) and get to the formula shown. What I've done so far is to draw free body diagrams of all the boxes, and then I've just tried to set up some equations and manipulate them. However, I can't seem to end up with the expression I'm supposed to end up with. I've attached my attempts at deriving a formula for S. The problem seems to be eliminating the acceleration-components. Any help would be very appreciated!