Answer
$3i\sqrt{2}-8i$
Work Step by Step
Using the Product Rule of radicals which is given by $\sqrt[m]{x}\cdot\sqrt[m]{y}=\sqrt[m]{xy},$ and $i=\sqrt{-1},$ the given expression is equivalent to
\begin{array}{l}\require{cancel}
\sqrt{-18}-\sqrt{-64}
\\\\=
\sqrt{-1}\cdot\sqrt{18}-\sqrt{-1}\cdot\sqrt{64}
\\\\=
i\sqrt{18}-i\sqrt{64}
.\end{array}
Extracting the root of the factor that is a perfect power of the index results to
\begin{array}{l}\require{cancel}
i\sqrt{18}-i\sqrt{64}
\\\\=
i\sqrt{9\cdot2}-i\sqrt{64}
\\\\=
i\sqrt{(3)^2\cdot2}-i\sqrt{(8)^2}
\\\\=
i\cdot3\sqrt{2}-i\cdot8
\\\\=
3i\sqrt{2}-8i
\\\\=
3i\sqrt{2}-8i
.\end{array}
