To multiply the fractions \( \frac{3}{7} \) and \( \frac{9}{10} \), you multiply the numerators together and the denominators together:
\[ \frac{3}{7} \times \frac{9}{10} = \frac{3 \times 9}{7 \times 10} = \frac{27}{70} \]
So, \( \frac{3}{7} \times \frac{9}{10} = \frac{27}{70} \).