A beam of proton projected towards +x axis and experience a magnetic force on -y axisFind directions of magnetic field

Using the right hand screw rule of cross product we can determine the direction of B.
v is along +X axis
F is along -Y axis
F = q ( V X B)
B must be along Z axis, but -Z or +Z?
If B is along +Z axis, then only will the direction of F (v cross B) will be along -Y axis:
Thus, as F is along -Y, B must be along +Z axis.