remember some basic rules
[tex](ab)^c=(a^c)(b^c)[/tex] and
[tex](a^b)^c=a^{bc}[/tex] and
[tex]x^{-m}= \frac{1}{x^m} [/tex]
so
[tex](3x^{-2}y^5)^4=((3x^{-2})^4)((y^5)^4)[/tex]
[tex](3x^{-2})^4=(3^4)((x^{-2}^4)=(81)(x^{-8})[/tex]
[tex](y^5)^4=y^{20}[/tex]
we now have
[tex]81x^{-8}y^{20}[/tex]
[tex]x^{-8}= \frac{1}{x^8} [/tex] so
[tex]81x^{-8}y^{20}= \frac{81y^{20}}{x^8} [/tex]
