\[y^{2} + by - 8 = 0;\ \ x_{1} = 4:\]
\[x_{1} \cdot x_{2} = - 8\]
\[x_{2} = - 2;\]
\[x_{1} + x_{2} = - b\]
\[- b = 4 - 2 = 2\]
\[b = - 2.\]
\[Ответ:x_{2} = - 2;b = - 2.\]