1. The problem statement, all variables and given/known data Obtain the accelerations of mass a and mass b. 2. Relevant equations Once i derive the displacement relation constraint, then i just have to diff twice w.r.t time. This part is ok. 3. The attempt at a solution I am trying to reason this way: suppose 2M moves up by x. Following the right string of 2M to pulley2 then to pulley 1 = This means pulley1 move down by x. Following the left string of 2M to pulley1 = this causes M to move down by x. So M moves by 2x. the acc of m must be 2a and acc of 2m is a. but the answer is m acc at 3a and 2m acc at a.