Appendix A - Review - A.1 Algebra Essentials - A.1 Assess Your Understanding - Page A11: 50

$d(D,B)=2$.

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