Answer
$DC = 18$
Work Step by Step
According to the perpendicular bisector theorem, points lying on the perpendicular bisector of a segment are equidistant from the segment's endpoints.
In the diagram, we see that $\overline{CE}$ is the perpendicular bisector of $\overline{BD}$; therefore, $C$ is equidistant from $B$ and $D$. $DC$, thus, is equal to $BC$, so if $BC = 18$, then $DC = 18$.