#### Answer

$-2x+y=0$

#### Work Step by Step

The result of exercise 32 tells us that ${\bf v}=a{\bf i} + b{\bf j}$
is parallel to the lines $bx-ay=c$
Here, ${\bf v}=-1\cdot{\bf i} + (-2)\cdot{\bf j}$,
$a=-1,\quad b=-2,$
and the lines are $-2x+y=c.$
$P(1,2)$ is on the line $\Rightarrow \left\{\begin{array}{l}
-2(1)+(2)=c\\
c=0
\end{array}\right.$
so the line equation is $\quad -2x+y=0$
(a second point on the line is $(0,0)$ )