\[\boxed{\mathbf{688\ (688).\ }Еуроки\ - \ ДЗ\ без\ мороки}\]
\[1)\ 2x² + 4x - b = 0\]
\[D = 16 - 4 \cdot 2 \cdot ( - b) = 0\]
\[16 + 8b = 0\]
\[b = - \frac{16}{8},\ \ b = - 2\]
\[Ответ:при\ b = - 2.\]
\[2)\ 3x² - bx + 12 = 0\]
\[D = b^{2} - 4 \cdot 3 \cdot 12 = 0\]
\[b^{2} = 144\]
\[b = 12;\ \ b = - 12\]
\[Ответ:\ при\ b = 12;\ \ b = - 12.\]