When a current flows through a wire that is placed in a magnetic field there is a force created due to the interaction of the field and the charges flowing through the wire.
The magnitude of this force is given by F = I*L*B where I is the current flowing through the wire, L is the length of the wire and B is the magnetic field.
In the problem, the length of the metal cross bar is 0.35 m, the current flowing through it is 3 A and a magnetic field of 1.2 T acts perpendicular to the cross bar and the rails. This results in a force along the rails equal to F = 3*0.35*1.2 = 1.26 N
But the crossbar moves at a constant velocity in spite of the force. This is due to frictional force between the crossbar and the rails that is equal to the force acting on it. The frictional force between the rails and the crossbar is 1.26 N.