$\left\{\begin{matrix}c=\frac{r}{7m}\text{, }&m\neq 0\\c\in \mathrm{C}\text{, }&r=0\text{ and }m=0\end{matrix}\right.$

$\left\{\begin{matrix}m=\frac{r}{7c}\text{, }&c\neq 0\\m\in \mathrm{C}\text{, }&r=0\text{ and }c=0\end{matrix}\right.$

$\left\{\begin{matrix}c=\frac{r}{7m}\text{, }&m\neq 0\\c\in \mathrm{R}\text{, }&r=0\text{ and }m=0\end{matrix}\right.$

$\left\{\begin{matrix}m=\frac{r}{7c}\text{, }&c\neq 0\\m\in \mathrm{R}\text{, }&r=0\text{ and }c=0\end{matrix}\right.$