Answer
$(3y)^{12}$
Work Step by Step
Since $a^x\cdot a^y=a^{x+y}$, then the given expression, $
(3y)^4\cdot (3y)^8
$, simplifies to
\begin{array}{l}\require{cancel}
(3y)^{4+8}
\\\\=
(3y)^{12}
.\end{array}
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