There are two forces acting on the box: the tension of the rope (upward) and the weight (downward). The problem says the box moves up at constant speed: this means that its acceleration is zero, therefore for Newton's second law, the resultant of the forces acting on the box is zero. This implies that the tension of the rope is equal to the weight of the box, which is:
[tex] T=W=mg=(50 kg)(9.81 m/s^2)=490.5 N [/tex]
