$\left\{\begin{matrix}a=\frac{a_{6}-6}{x}\text{, }&x\neq 0\\a\in \mathrm{C}\text{, }&a_{6}=6\text{ and }x=0\end{matrix}\right.$

$\left\{\begin{matrix}a=\frac{a_{6}-6}{x}\text{, }&x\neq 0\\a\in \mathrm{R}\text{, }&a_{6}=6\text{ and }x=0\end{matrix}\right.$

$a_{6}=ax+6$