\(\frac{21}{5}\cdot\frac{3}{7}\)

Сократим 21 и 7 на 7: \(\frac{21}{5}\cdot\frac{3}{7}=\frac{3}{5}\cdot\frac{3}{1}=\frac{9}{5}=1\frac{4}{5}\).

Ответ: \(\frac{9}{5}=1\frac{4}{5}\).