$$y = m x + c : 7 - 3 x + 4 y = 0 : 20$$
$y=mx+\frac{c}{7}+4y-3x\text{ and }mx+\frac{c}{7}+4y-3x=0$
$\left\{\begin{matrix}x=-\frac{c}{7\left(m-3\right)}\text{, }&y=0\text{ and }m\neq 3\\x\in \mathrm{R}\text{, }&y=0\text{ and }m=3\text{ and }c=0\end{matrix}\right.$
$y=0$
$c=7x\left(3-m\right)$
