Ответ:

а) \[\frac{6}{7}-\frac{2}{3}=\frac{18}{21}-\frac{14}{21}=\frac{4}{21}.\]

б) \[\frac{11}{12}-\frac{5}{6}=\frac{11}{12}-\frac{10}{12}=\frac{1}{12}.\]

в) \[\frac{7}{12}-\frac{3}{8}=\frac{14}{24}-\frac{9}{24}=\frac{5}{24}.\]

г) \[\frac{9}{14}-\frac{19}{35}=\frac{45}{70}-\frac{38}{70}=\frac{7}{70}=\frac{1}{10}.\]

Ответ: а) \(\frac{4}{21}\); б) \(\frac{1}{12}\); в) \(\frac{5}{24}\); г) \(\frac{1}{10}\).