A box of mass 1kg is placed on a table and is connected to a hanging mass m by a string strung over a pulley. If the coefficient of kinetic friction between the box and the table is .3 and the box slides at a constant speed of .5m/s what is the weight of the hanging mass?

F_box=Fg of hanging mass
ma=mg
(1kg)a=m_hanging(9.8m/s^2)

But the problem is that i'm not given the acceleration of the 1kg box. How would i solve this problem?