Решение:
а) $-\frac{1}{5} + \frac{6}{25} - \frac{8}{25} = -\frac{5}{25} + \frac{6}{25} - \frac{8}{25} = -(\frac{5}{25} + \frac{8}{25}) + \frac{6}{25} = -\frac{13}{25} + \frac{6}{25} = -\frac{7}{25}$;
б) $-\frac{1}{7} + \frac{2}{21} - \frac{3}{7} = -\frac{3}{21} + \frac{2}{21} - \frac{9}{21} = -(\frac{3}{21} + \frac{9}{21}) +\frac{2}{21} = -\frac{12}{21} + \frac{2}{21} = -\frac{10}{21}$;
в) $-\frac{8}{49} - \frac{5}{7} - \frac{9}{49} = -(\frac{8}{49} + \frac{35}{49} + \frac{9}{49}) = -\frac{52}{49} = -1\frac{3}{49}$;
г) $\frac{7}{10} - \frac{4}{15} - \frac{11}{30} = \frac{21}{30} - \frac{8}{30} - \frac{11}{30} = \frac{21 - 8 - 11}{30} = \frac{2}{30} = \frac{1}{15}$.
Ответ:
а) $-\frac{7}{25}$;
б) $-\frac{10}{21}$;
в) $-1\frac{3}{49}$;
г) $\frac{1}{15}$.