Answer
$d(D, E) = 2$
Work Step by Step
$d(D, E)$ represents the distance between the points D and E on a number line.
If the coordinates of D and E are $d$ and $e$, respectively, then:
$d(D, E) = |d-e|$
Based on the given number line, the coordinates of D and E are 1 and $3$, respectively.
Use the formula above to obtain:
$d(D, E) = |1-3| = |-2| =2$
Thus,
$d(D, E) = 2$
