1. The problem statement, all variables and given/known data Given Ma = Mb =Mc and muk between c and the surface is 0.30 2. Relevant equations (rope length) = constant Maaa = Mag - T Mbab = Mbg - 2T Mcac = T - (mu)Mcg 3. The attempt at a solution The equations I came up on my own and the answer key did as well. The problem is I also need an equation relating the length of the rope as constant. My convention is to measure the length of the rope in the direction of positive motion as defined by my equations. In this case I make the assumption mass A and mass B accelerate down (the answer later gives B as negative indicating it goes up) and made that the positive direction. I also made going to the left positive for mass C. The problem is when I go to write the rope length equation I get Xa + 2 Xb + (X - Xc) = constant and obviously differentiate to get the acceleration equation. I measured X as the distance from the right side to the pulley nearest C and thus X - Xc is the rope length between that pulley and C. This is so that I measure positive distances in the direction of positive acceleration. The answer key simply lists the equation as Xa + 2 Xb + Xc) = constant with all other equations I gave as true. Where am I picturing this wrong?