Решение:

a) $$rac{x}{5} + \frac{2x}{3} = \frac{3x}{15} + \frac{10x}{15} = \frac{3x + 10x}{15} = \frac{13x}{15}$$

Ответ: $$rac{13x}{15}$$

б) $$rac{3b}{7} - \frac{b}{4} = \frac{12b}{28} - \frac{7b}{28} = \frac{12b - 7b}{28} = \frac{5b}{28}$$

Ответ: $$rac{5b}{28}$$

в) $$rac{6m}{7} - \frac{m}{11} = \frac{66m}{77} - \frac{7m}{77} = \frac{66m - 7m}{77} = \frac{59m}{77}$$

Ответ: $$rac{59m}{77}$$

г) $$rac{m}{42} + \frac{5m}{6} = \frac{m}{42} + \frac{35m}{42} = \frac{m + 35m}{42} = \frac{36m}{42} = \frac{6m}{7}$$

Ответ: $$rac{6m}{7}$$