Answer
$-2x-y=0$
Work Step by Step
The result of exercise 31 tells us that ${\bf v}=a{\bf i} + b{\bf j}$
is perpendicular to the lines $ax+by=c$
Here, ${\bf v}=-2\cdot{\bf i} + (-1)\cdot{\bf j}$,
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 $(0.0)$ )
$-2x-y=0$
