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

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