Atwood Machine. What is the final speed?

An atwood machine(consisting of mass m attached to the left side of the string and mass 2m attached to the right side of the string)has a third mass m attached to mass m by a limp string.After being released,the 2m mass falls a distance x before the limp string becomes taut.Thereafter both the masses on the left rise with the same speed.What is the final speed?Assume that pulley is ideal.