Answer
$\frac{27}{a^{12}b^{21}}$
Work Step by Step
$(\frac{3a^{-4}}{b^{7}})^{3}$
=$(\frac{3}{a^{4}b^{7}})^{3}$
=$(\frac{3^{3}}{(a^{4})^{3}(b^{7})^{3}})$
=$\frac{27}{a^{12}b^{21}}$
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