Given equation of the line is $l x +m y +n=0$
$\Rightarrow m y =-l x-n$
$\Rightarrow y =\frac{-l}{m} x-\frac{n}{m}$
On comparing with $y=M x +C$, we get
$M=\frac{-l}{m}$ and $C=\frac{-n}{m}$
Again, given equation of the parabola is $y^{2}=8 x$
On comparing with $y^{2}=4 a x$, we get
$4 a=8 \Rightarrow a=2$
Now, by condition of tangency,
$C=\frac{a}{M}$
$\Rightarrow \frac{-n}{m}=\frac{2}{-1 / m}$ [from Eq.(i)]
$\Rightarrow \frac{n}{m}=\frac{2 m}{l} \Rightarrow \ln =2 m^{2}$