Answer
$x+2y=4$
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, ${\bf v}=1\cdot{\bf i} + 2\cdot{\bf j}$, and the lines are $x+2y=c.$
P(2,1) is on the line $\Rightarrow \left\{\begin{array}{l}
(2)+2(1)=c\\
c=4
\end{array}\right.$
so the line equation is $\quad x+2y=$4
(intercepts: $(4,0)$ and $(0.2)$ )
