Answer
$\dfrac{9y^6}{x^8}$
Work Step by Step
RECALL:
(i) $(a^m)^n = a^{mn}$
(ii) $(ab)^m=a^mb^m$
(iii) $a^{-m} = \dfrac{1}{a^m}, a\ne0$
Use the rules above to have:
$=3^2(x^{-4})^2(y^3)^2
\\=9x^{-4(2)}y^{3(2)}
\\=9x^{-8}y^6
\\=9y^6 \cdot \dfrac{1}{x^8}
\\=\dfrac{9y^6}{x^8}$