According to the Fleming's left handrule we know that when a positive charge moves with velocity v perpendicular to the direction of magnetic field the it experiences a magnetic force perpendicular to the the plane in which the velocity vector and the magnetic field vector lies.
As a result all the three vector are mutually perpendicular in such a case.
Here as per given condition we have a negative charge moving along x-axis which experiences the deflection in -y direction so the magnetic field will be along the negative z axis i.e. into the screen.
