\[\boxed{\mathbf{868\ (868)}\mathbf{.}\mathbf{\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[\textbf{а)}\ + ( - 2,4) = - 2,4;\]
\[+ ( + 0,6) = 0,6;\]
\[+ \left( - \frac{5}{7} \right) = - \frac{5}{7}.\]
\[\textbf{б)} - (1,7) = - 1,7;\]
\[- ( - 8,5) = + 8,5 = 8,5;\]
\[- \left( - \frac{2}{9} \right) = + \frac{2}{9} = \frac{2}{9}.\]