в) \(6\frac{15}{21}+2\frac{9}{14}\);

Сократим первую дробь: \(\frac{15}{21}=\frac{5}{7}=\frac{10}{14}\). Складываем:

\(6\frac{15}{21}+2\frac{9}{14}=8+\frac{10}{14}+\frac{9}{14}=8+\frac{19}{14}=9\frac{5}{14}.\)

Ответ: \(9\frac{5}{14}\).