Ответ:

а) \(\frac{4}{9}\cdot\left(-\frac{3}{8}\right)=-\frac{12}{72}=-\frac{1}{6}\).

б) \(\frac{5}{6}-\frac{4}{9}:\frac{16}{21}=\frac{5}{6}-\frac{4}{9}\cdot\frac{21}{16}=\frac{5}{6}-\frac{7}{12}=\frac{10}{12}-\frac{7}{12}=\frac{1}{4}\).

Ответ: а) \(-\frac{1}{6}\); б) \(\frac{1}{4}\).