Answer
$-2$
Work Step by Step
The slope of a line perpendicular to a line $l$ is equal to the negative reciprocal of the slope of $l$.
We find slope by choosing two points on line $l$ and using the formula: $slope = \frac{y_2-y_1}{x_2-x_1}$
Slope of segment $CD = \frac{4-2}{5-1} = \frac{2}{4} = \frac{1}{2}$. The negative reciprocal of $\frac{1}{2} = -\frac{2}{1} = -2$
