An electron is moving along the positive x-axis. A uniform magnetic field is switched on for a short time so that the electron reverses its direction of motion and starts moving parallel to the negative x-axis. Then the magnetic field is applied along

To reverse the direction of motion, the electron must rotate through half circle. If the electron rotates in xy-plane, magnetic field must be applied along $\pm$z axis. If the electron rotates in z-x plane, the magnetic field must be applied along $\pm$ y axis.