7. a) -3/8 * (4/9); б) -7/15 * 3/5.

а) \(-\frac{3}{8} \cdot \frac{4}{9} = -\frac{3 \cdot 4}{8 \cdot 9} = -\frac{12}{72} = -\frac{1}{6}\). б) \(-\frac{7}{15} \cdot \frac{3}{5} = -\frac{7 \cdot 3}{15 \cdot 5} = -\frac{21}{75} = -\frac{7}{25}\).