\[y = kx + b.\]
\[A( - 3;0);\ \ B(0; - 1).\]
\[- 1 = 0 \cdot k + b\]
\[b = - 1.\]
\[0 = - 3k - 1\]
\[- 3k = 1\]
\[k = - \frac{1}{3}.\]
\[Формула\ функции:y = - \frac{1}{3}x - 1.\]