The perpendicular from the origin to a line $L$ meets it at the point $(3, – 9),$ equation of the line $L$ is

Slope of the perpendicular line is $\frac{-9}{3}=-3$
$\Rightarrow$slope of $L$ is $\left(1/3\right)$ and as it passes through $(3, -9)$  equation of $L$ is 

$\begin{array}{l} y+9=(1/3)(x-3)\\ \Rightarrow x-3y=30\end{array}$