The equations $3x-7y+k=0$ and  $12x-\ell y+36=0$ have infinitely many solutions if

$$\begin{gathered} 3x-7y+k=0 ,12x-\ell y+36=0 have infinitely many solutions \\ \Rightarrow \frac{3}{12}=\frac{-7}{-\ell }=\frac{k}{36}\Rightarrow \ell =28,k=9 \\ \left[formula \frac{a_1}{a_2}=\frac{b_1}{b_2}=\frac{c_1}{c_2}\right] \end{gathered}$$