\( \frac{2}{3} - b = \frac{5}{6} \)
\( b = \frac{2}{3} - \frac{5}{6} \)
\( b = \frac{4}{6} - \frac{5}{6} \)
\( b = -\frac{1}{6} \)
Ответ: \( b = -\frac{1}{6} \)