Answer
$-j.$
Work Step by Step
If $v=ai+bj+ck$ and $||v||=\sqrt{a^2+b^2+c^2}$, then the unit vector $u$ in the same direction as $v$ is
$u=\frac{v}{||v||}$.
The magnitude of $v$ is
$||v||=\sqrt{0^2+(-3)^2+0^2}\\||v||=\sqrt{0+9+0}\\||v||=\sqrt{9}=3.$
Thus the unit vector is: $u=\frac{-3j}{3}\\u=-j.$
