$a)\frac{6}{7}-\frac{3}{4}=\frac{24}{28}-\frac{21}{28}=\frac{3}{28}$

$b)\frac{12}{13}-\frac{2}{5}=\frac{60}{65}-\frac{36}{65}=\frac{24}{65}$ $b)\frac{12}{13}-\frac{2}{5}=\frac{60}{65}-\frac{36}{65}=\frac{24}{65}$

$c)1\frac{2}{3}-\frac{1}{6}=1\frac{4}{6}-\frac{1}{6}=1\frac{3}{6}=1\frac{1}{2}$

$d)2\frac{1}{2}-1\frac{3}{4}=2\frac{2}{4}-1\frac{3}{4}=1\frac{6}{4}-1\frac{3}{4}=\frac{3}{4}$

$e)12\frac{7}{10}-8\frac{7}{8}=12\frac{28}{40}-8\frac{35}{40}=11\frac{68}{40}-8\frac{35}{40}=3\frac{33}{40}$

$f)4\frac{7}{9}-2\frac{11}{12}=4\frac{28}{36}-2\frac{33}{36}=3\frac{64}{36}-2\frac{33}{36}=1\frac{31}{36}$