\[ \left( \frac{3}{7} + \left( -\frac{8}{7} \right) \right) + 3 \frac{6}{7} = \left( \frac{3}{7} - \frac{8}{7} \right) + \frac{27}{7} = \frac{-5}{7} + \frac{27}{7} = \frac{-5 + 27}{7} = \frac{22}{7} = 3 \frac{1}{7} \]
Ответ: 3 1/7.