\[\boxed{\text{168\ (168).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[= 3a - 2b\]
\[при\ a = \frac{2}{3};\ \ b = - \frac{1}{2}:\]
\[3 \cdot \frac{2}{3} - 2 \cdot \left( - \frac{1}{2} \right) = 2 + 1 = 3.\]
\[= \frac{a - 5b}{5} \cdot \frac{25}{(a - 5b)(a + 5b)} =\]
\[= \frac{5}{a + 5b}\]
\[при\ a = - 8;b = 0,6:\]
\[\frac{5}{- 8 + 5 \cdot 0,6} = \frac{5}{- 8 + 3} = \frac{5}{- 5} =\]
\[= - 1.\]