к) $$ 3 \frac{1}{8} \cdot \frac{21}{9} = \frac{25}{8} \cdot \frac{21}{9} = \frac{25 \cdot 21}{8 \cdot 9} = \frac{525}{72} = \frac{175}{24} = 7 \frac{7}{24} $$
Ответ: $$7 \frac{7}{24}$$