$D=-12b\left(ac\right)^{3}$

$\left\{\begin{matrix}a=-\frac{12^{\frac{2}{3}}\sqrt[3]{\frac{D}{b}}}{12c}\text{, }&c\neq 0\text{ and }b\neq 0\\a\in \mathrm{R}\text{, }&\left(b=0\text{ or }c=0\right)\text{ and }D=0\end{matrix}\right.$