$${a = -g \frac{2m}{M + 2m} = -(9,8\, \mathrm{m/s^2}) \frac{(2)(1,2\, \mathrm{kg})}{2,5\, \mathrm{kg} + (2)(1,2\, \mathrm{kg})} \\ = \underline{\underline{-4,8\, \mathrm{m/s^2}}}}$$