\[\boxed{\text{131\ (131).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[3x = a - b\]
\[x = \frac{a - b}{3}\]
\[- 7x = a - b - b\]
\[- 7x = a - 2b\]
\[x = - \frac{a - 2b}{- 7}\]
\[x = \frac{2b - a}{7}\]
\[\frac{x}{a} = b - 1\]
\[x = a \cdot (b - 1)\]
\[\frac{x}{10} = b - a\]
\[x = 10(b - a)\ \]