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