Answer
$(\sqrt {-72})^3=-432\sqrt 2i$
Work Step by Step
Use: $i^2=-1$ and $\sqrt {-1}=i$
$(\sqrt {-72})^3=(\sqrt {72}\sqrt {-1})^3=(6\sqrt 2i)^3=(6\sqrt 2)^3i^3=6^3(\sqrt 2)^3(i^2)i=216(2\sqrt 2)(-1)i=-432\sqrt 2i$
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