An electron is moving along positive x-axis. To get it moving on an anticlockwise circular path in x-y plane, a magnetic field is applied

To move the electron in xy plane, force on it must be acting in the y-direction initially. The direction of F→ is known, and the direction of v is known, hence by applying Fleming's left hand rule, the direction of magnetic field is also determined. Also,j^=−ei^  ×  k^