$$d=(2a)^{2}-4X1X-(4l^{2}-a^{2})$$
$\left\{\begin{matrix}X=\frac{5a^{2}-d-4l^{2}}{4X_{1}}\text{, }&X_{1}\neq 0\\X\in \mathrm{R}\text{, }&d=5a^{2}-4l^{2}\text{ and }X_{1}=0\end{matrix}\right.$
$\left\{\begin{matrix}X_{1}=\frac{5a^{2}-d-4l^{2}}{4X}\text{, }&X\neq 0\\X_{1}\in \mathrm{R}\text{, }&d=5a^{2}-4l^{2}\text{ and }X=0\end{matrix}\right.$