Answer
$8y^{4}\sqrt y$
Work Step by Step
$\sqrt (64y^{9})=\sqrt (64y^{8}\times y)=\sqrt (64y^{8})\times\sqrt y=8y^{4}\sqrt y$
We know that $\sqrt (64y^{8})=8y^{4}$, because $(8y^{4})^{2}=(8\times8)\times y^{4+4}=64y^{8}$.
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