Решим каждый пример по действиям:
a) $$6\frac{5}{12} + 3\frac{3}{8} = 6\frac{10}{24} + 3\frac{9}{24} = 9\frac{19}{24}$$
б) $$2\frac{11}{20} + 4\frac{19}{30} = 2\frac{33}{60} + 4\frac{38}{60} = 6\frac{71}{60} = 7\frac{11}{60}$$
в) $$3 - \frac{18}{19} = \frac{3 \cdot 19}{19} - \frac{18}{19} = \frac{57}{19} - \frac{18}{19} = \frac{39}{19} = 2\frac{1}{19}$$
г) $$5\frac{6}{7} - 3\frac{1}{14} = 5\frac{12}{14} - 3\frac{1}{14} = 2\frac{11}{14}$$
Ответ: