\[\boxed{\mathbf{721\ (721).\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[x^{2} + bx - 17 = 0\]
\[x_{1} \cdot x_{2} = - 17\]
\[x_{1} + x_{2} = - b\]
\[x_{1} = - x_{2}\]
\[- x_{2}^{2} = - 17\]
\[x_{2}^{2} = 17\]
\[x_{2} = \sqrt{17};\ \ x_{1} = - \sqrt{17}\]
\[\sqrt{17} - \sqrt{17} = - b = 0\]
\[Ответ:b = 0.\]