Б) (x - 9\(\frac{3}{7}\)) + 5\(\frac{8}{21}\) = 6\(\frac{5}{14}\).

(x - 9\(\frac{3}{7}\)) + 5\(\frac{8}{21}\) = 6\(\frac{5}{14}\)

x - 9\(\frac{3}{7}\) = 6\(\frac{5}{14}\) - 5\(\frac{8}{21}\)

x - 9\(\frac{3}{7}\) = (6-5) + (\(\frac{5}{14}\) - \(\frac{8}{21}\)) = 1 + (\(\frac{5 \cdot 3}{14 \cdot 3}\) - \(\frac{8 \cdot 2}{21 \cdot 2}\)) = 1 + \(\frac{15}{42}\) - \(\frac{16}{42}\) = 1 - \(\frac{1}{42}\) = \(\frac{42}{42}\) - \(\frac{1}{42}\) = \(\frac{41}{42}\)

x = \(\frac{41}{42}\) + 9\(\frac{3}{7}\) = 9 + (\(\frac{41}{42}\) + \(\frac{3 \cdot 6}{7 \cdot 6}\)) = 9 + \(\frac{41}{42}\) + \(\frac{18}{42}\) = 9 + \(\frac{59}{42}\) = 9 + 1\(\frac{17}{42}\) = 10\(\frac{17}{42}\)

Ответ: 10\(\frac{17}{42}\)