a.

$\frac{3}{5} + \frac{2}{7}=\frac{21}{35} + \frac{10}{35}=\frac{31}{35}$

$\frac{7}{9} - \frac{1}{6} =\frac{14}{18} - \frac{3}{18}=\frac{11}{18}$

$\frac{7}{8} + 3 = \frac{7}{8} + \frac{24}{8}=\frac{31}{8}$

$2 - \frac{5}{4}=\frac{8}{4} - \frac{5}{4}=\frac{3}{4}$

b. 

$\frac{9}{2}\times \frac{4}{5}=\frac{9\times 4}{2\times 5}=\frac{36}{10}$

$\frac{8}{7} :\frac{2}{3}=\frac{8}{7}\times \frac{3}{2}=\frac{24}{14} = \frac{12}{7}$

$4\times \frac{5}{9}=\frac{4\times 5}{1\times 9}=\frac{20}{9}$

$\frac{3}{8} : 2 = \frac{3}{8}\times \frac{1}{2}=\frac{3}{16}$