[tex]\\ \rm\Rrightarrow (x^{2/3}y^{1/2})^{-6}=x^{2(-2)}y^{-3}=x^{-4}y^{-3}=\dfrac{1}{x^4y^3}[/tex]
#2
[tex]\\ \rm\Rrightarrow 4^{3/4}4^{-1/2}=4^{3/4-1/2}=4^{3-2/4}=4^{1/4}=\sqrt[4]{4}=\sqrt{2}[/tex]
#3
[tex]\\ \rm\Rrightarrow (\dfrac{6x^6y^8}{12^{16}})^{1/4}[/tex]
[tex]\\ \rm\Rrightarrow \dfrac{6^{1/4}x^{3/2}y^{2}}{12^4}[/tex]
