\[y = - \frac{1}{5}x + b;\ \ M\ (20;\ - 3):\]
\[- \frac{1}{5} \cdot 20 + b = - 3\]
\[- 4 + b = - 3\]
\[b = 1.\]
\[Ответ:при\ b = 1.\ \]