$\left\{\begin{matrix}a=2x-b\text{; }a=0\text{, }&x\neq b\text{ and }x\neq 0\\a\neq x\text{, }&b=0\text{ and }x\neq 0\end{matrix}\right.$

$\left\{\begin{matrix}b=-\left(a-2x\right)\text{; }b=0\text{, }&x\neq 0\text{ and }a\neq x\\b\neq x\text{, }&a=0\text{ and }x\neq 0\end{matrix}\right.$