Answer
$-j.$
Work Step by Step
If $v=ai+bj$ then the unit vector $u$ in the direction of $v$ is $u=\frac{v}{||v||}$, where $||v||=\sqrt{a^2+b^2}.$ $||v||=\sqrt{(-3)^2+0^2}=\sqrt{9}=3.$
Hence $u=\frac{-3j}{3}=-j.$
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