а) Найдите значение выражения: \(3\frac{1}{6}+\left(-2\frac{4}{9}-a\right)\), если \(a=-1\frac{2}{3}\); б) \(\left(2\frac{5}{8}-3\frac{1}{6}\right)-(-1{,}85-4{,}4)-5\frac{1}{24}\).

а) Подставим значение \(a\):

\[3\frac{1}{6}+\left(-2\frac{4}{9}+1\frac{2}{3}\right)=\frac{19}{6}+\left(-\frac{22}{9}+\frac{5}{3}\right).\]

\[\frac{19}{6}+\left(-\frac{22}{9}+\frac{15}{9}\right)=\frac{19}{6}-\frac{7}{9}=\frac{57-14}{18}=\frac{43}{18}=2\frac{7}{18}.\]

Ответ: \(2\frac{7}{18}\).

б) \[\left(2\frac{5}{8}-3\frac{1}{6}\right)-(-1{,}85-4{,}4)-5\frac{1}{24}=\left(\frac{21}{8}-\frac{19}{6}\right)+6{,}25-\frac{121}{24}.\]

\[\frac{21}{8}-\frac{19}{6}=\frac{63-76}{24}=-\frac{13}{24},\qquad 6{,}25=\frac{25}{4}=\frac{150}{24}.\]

\[-\frac{13}{24}+\frac{150}{24}-\frac{121}{24}=\frac{16}{24}=\frac{2}{3}.\]

Ответ: \(\frac{2}{3}\).