\[\boxed{\mathbf{764\ (764)}.\ Еуроки\ - \ ДЗ\ без\ мороки}\]
\[1)\ \frac{b}{7} = 12\]
\[b = 12 \cdot 7\]
\[b = 84\]
\[Ответ:84.\]
\[2)\frac{169}{m} = 13\]
\[m = \frac{169}{13}\]
\[m = 13\]
\[Ответ:13.\]
\[3)\frac{126}{8 - y} = 21\]
\[8 - y = \frac{126}{21}\]
\[8 - y = 6\]
\[y = 8 - 6\]
\[y = 2\]
\[Ответ:2.\]