Problem

The key is using the Lorentz force law for a moving charged particle: $\vec{F} = q\vec{v} \times \vec{B}$. Since the electron moves in the $+x$ direction and experiences a force in the $+y$ direction, you need to find which magnetic field direction makes that cross product work out—but remember the electron's negative charge flips the direction of the force relative to the cross product. Work backwards from the desired force direction to find $\vec{B}$.

The key is using the Lorentz force law for a moving charged particle: $\vec{F} = q\vec{v} \times \vec{B}$. Since the electron moves in the $+x$ direction and experiences a force in the $+y$ direction, you need to find which magnetic field direction makes that cross product work out—but remember the electron's negative charge flips the direction of the force relative to the cross product. Work backwards from the desired force direction to find $\vec{B}$.