Walker, Halliday & Resnick — Force and Motion-I: Problem 13

13 Two particles of masses $m$ and $2m$ are placed on a smooth horizontal table. A string, which joins these two masses, hangs over the edge supporting a pulley, which suspends a particle of mass $3m$, as shown in Fig. 5-23. The pulley has negligible mass. The two parts of the string on the table are parallel and perpendicular to the edge of the table. The hanging parts of the string are vertical. Find the acceleration of the particle of mass $3m$.