Сокращаем каждую дробь:
\frac{14}{21} = \frac{2 \cdot 7}{3 \cdot 7} = \frac{2}{3}
\frac{6}{9} = \frac{2 \cdot 3}{3 \cdot 3} = \frac{2}{3}
\frac{88}{99} = \frac{8 \cdot 11}{9 \cdot 11} = \frac{8}{9}
\frac{70}{84} = \frac{5 \cdot 14}{6 \cdot 14} = \frac{5}{6}