\[bx² - 3x - 7 = 0\]
\[D = ( - 3)^{2} - 4 \cdot b \cdot ( - 7) =\]
\[= 9 + 28b\]
\[9 + 28b = 0\]
\[28b = - 9\ \]
\[b = - \frac{9}{28}\]
\[Ответ:\ при\ b = - \frac{9}{28};b = 0.\]