Answer
$+z$
Work Step by Step
We know that $q $ is positive and we want $\vec{v}\times\vec{B}$ to have
the same direction as $\vec{F}$ , which is $-\mathrm{y}$.
Now, we will use the right hand rule (see fig. 3.19 on p. 53) as follows:
Outstretch your right arm and turn your hand so your thumb points down.
The fingers are pointing to the right (the fingers represent $\vec{v}$ in the $+x$ direction, and the thumb $\vec{v}\times\vec{B}$ in the $-y$ direction).
Swipe your fingers roughly for a right angle. The fingers now point behind you;
This is the +z direction, when compared to the diagram.
This is where $\vec{B}$ points.
