$\left\{\begin{matrix}\\a=x\left(x+3\right)\text{, }&\text{unconditionally}\\a=-\frac{x\left(3-x\right)}{3}\text{, }&x\leq -6\text{ or }x\geq 0\\a=-\frac{x\left(3-x\right)}{3}\text{, }&x\geq -6\text{ and }x\leq 0\end{matrix}\right.$

$\left\{\begin{matrix}x=\frac{\sqrt{12a+9}+3}{2}\text{; }x=\frac{-\sqrt{12a+9}+3}{2}\text{, }&a\geq -\frac{3}{4}\\x=\frac{-\sqrt{4a+9}-3}{2}\text{; }x=\frac{\sqrt{4a+9}-3}{2}\text{, }&a\geq -\frac{9}{4}\end{matrix}\right.$