\[27^{- 3}\ :9^{- 4} = \left( 3^{3} \right)^{- 3}\ :\left( 3^{2} \right)^{- 4} =\]
\[= 3^{- 9}\ :3^{- 8} = 3^{- 1} = \frac{1}{3}\]