\[\boxed{\mathbf{57}\mathbf{.}}\]
\[1)\ 64^{\frac{1}{2}} = \left( 8^{2} \right)^{\frac{1}{2}} = 8\]
\[2)\ 27^{\frac{1}{3}} = \left( 3^{3} \right)^{\frac{1}{3}} = 3\]
\[3)\ 8^{\frac{2}{3}} = \left( 2^{3} \right)^{\frac{2}{3}} = 2^{2} = 4\]
\[4)\ 81^{\frac{3}{4}} = \left( 3^{4} \right)^{\frac{3}{4}} = 3^{3} = 27\]
\[5)\ 16^{- 0,75} = 16^{- \frac{3}{4}} = \left( 2^{4} \right)^{- \frac{3}{4}} =\]
\[= 2^{- 3} = \frac{1}{2^{3}} = \frac{1}{8}\]
\[6)\ 9^{- 1,5} = 9^{- \frac{3}{2}} = \left( 3^{2} \right)^{- \frac{3}{2}} = 3^{- 3} =\]
\[= \frac{1}{3^{3}} = \frac{1}{27}\]