Answer
$y=-\frac{1}{2}x+\frac{5}{2}$ or $x+2y=5$
Work Step by Step
Step 1. Given $L$ is perpendicular to $y=2x, (m'=2)$, we can find the slope $m=-\frac{1}{m'}=-\frac{1}{2}$.
Step 2. Using the point $(1,2)$, we can write the equation as $y-2=-\frac{1}{2}(x-1)$ or $y=-\frac{1}{2}x+\frac{5}{2}$ or $x+2y=5$
