a)
(5^{-\frac{1}{2}}*5^{\frac{2}{3}})^3=(5^{-\frac{3}{6}+\frac{4}{6}})^3=5^{\frac{1}{6}*3}=5^{\frac{1}{2}}=\sqrt5
b)
\frac{2,5^{4,5}*2,5^{-3}}{0,4^{-0,5}*0,4^{-\frac{3}{4}}}*6,4^{\frac{1}{4}}=
\frac{2,5^{4,5-3}}{0,4^{-\frac{1}{2}-\frac{3}{4}}}*(\frac{64}{10})^{\frac{1}{4}}=
\frac{2,5^{1,5}}{(\frac{4}{10})^{\frac{-2-3}{4}}}*(\frac{32}{5})^{\frac{1}{4}}=
\frac{2,5^{1,5}}{(\frac{2}{5})^{-\frac{5}{4}}}*\frac{(2^5)^{\frac{1}{4}}}{5^{\frac{1}{4}}}=\frac{(\frac{5}{2})^{\frac{3}{2}}}{(\frac{5}{2})^{\frac{5}{4}}}*\frac{2^{\frac{5}{4}}}{5^{\frac{1}{4}}}=
(\frac{5}{2})^{\frac{6}{4}-\frac{5}{4}}*\frac{2^1*2^{\frac{1}{4}}}{5^{\frac{1}{4}}}=\frac{5^{\frac{1}{4}}}{2^{\frac{1}{4}}}*\frac{2*2^{\frac{1}{4}}}{5^{\frac{1}{4}}}=2