5. In a regular tetrahedron $ABCD$, $E$ and $F$ are on edges $AB$ and $AC$, respectively, such that $BE=3$, $EF=4$, and $EF$ is parallel to plane $BCD$. Then the area of $\triangle DEF$ is $\qquad$
$E F // $ plane $B C D \Rightarrow \triangle A E F$ is an equilateral triangle $\Rightarrow A C=7 \Rightarrow \overrightarrow{A F}=\frac{4}{7} \overrightarrow{A C}$ $\Rightarrow \overrightarrow{D F}=\frac{3}{7} \overrightarrow{D A}+\frac{4}{7} \overrightarrow{D C} \Rightarrow \overrightarrow{D F}^{2}=9+16+3 \times 4=37 \Rightarrow D F=\sqrt{37}$, then the height $h$ on the base $E F$ of $\triangle D E F$ is $h=\sqrt{37-4}=\sqrt{33} \Rightarrow S_{\triangle D E F}=2 \sqrt{33}$.