56. \(-\frac{2}{7} - (-\frac{8}{21})\)

Решение:

\(-\frac{2}{7} - (-\frac{8}{21}) = -\frac{2}{7} + \frac{8}{21} = -\frac{2 \times 3}{7 \times 3} + \frac{8}{21} = -\frac{6}{21} + \frac{8}{21} = \frac{-6 + 8}{21} = \frac{2}{21}\)

Ответ: \(\frac{2}{21}\)