Answer
$-3i\sqrt 2$
Work Step by Step
$-\sqrt -18=-(\sqrt (9\times2\times-1))=-(\sqrt 9\times\sqrt 2\times\sqrt -1)=-(3i\sqrt 2)=-3i\sqrt 2$
We know that $\sqrt 9=3$, because $3^{2}=9$.
We know that $\sqrt -1=i$, because $i^{2}=-1$.
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