Appendix A - Review - A.1 Algebra Essentials - A.1 Assess Your Understanding - Page A11: 47

$d(D,E)=2$.

The coordinate of point $D$ is $=1$ The coordinate of point $E$ is $=3$ The distance between $D$ and $E$ is $d(D,E)=\left | 3-1 \right |$ Simplify. $d(D,E)=\left | 2 \right |$ $d(D,E)=2$ Hence, the distance between $D$ and $E$ is $2$.