10. As shown in the figure, in isosceles $\triangle ABC$, $AB=AC$, point $P$ is on the altitude $AD$ of $BC$, and $\frac{AP}{PD}=\frac{1}{3}$. The extension of $BP$ intersects $AC$ at $E$. If $S_{\triangle ABC}=21$, then $S_{\triangle DEC}=$ $\qquad$ .
$9$