$$M=y_{2}-y_{1}; x_{2}-x_{1}$$
$M=-\frac{y_{1}-y_{2}}{x_{2}-x_{1}}$
$x_{2}\neq x_{1}$
$\left\{\begin{matrix}x_{1}=\frac{Mx_{2}+y_{1}-y_{2}}{M}\text{, }&y_{2}\neq y_{1}\text{ and }M\neq 0\\x_{1}\neq x_{2}\text{, }&M=0\text{ and }y_{2}=y_{1}\end{matrix}\right.$
