AIPMT 2014 Physics and Chemistry Paper

A system consists of three masses m1,m2 and m3 connected by a string passing over a pulley P. The mass m1 hangs freely and m2 and m3 are on a rough horizontal table (the coefficient of friction =µ ). The pulley is frictionless and of negligible mass. The downward acceleration of mass m1 is :
(Assume m1=m2=m3=m )