6. $P$ is the midpoint of the height $V O$ of the regular quadrilateral pyramid $V-A B C D$. If the distance from point $P$ to the side face is 3, and the distance from point $P$ to the base is 5, then the volume of the regular quadrilateral pyramid is
$$
\begin{array}{l}
\text { As shown, } V P=5, P H=3 \Rightarrow V H=4, \\
\triangle V P H \sim \triangle V E O \Rightarrow \frac{V H}{P H}=\frac{V O}{E O} \\
\Rightarrow O E=\frac{15}{2} \Rightarrow A B=15 .
\end{array}
$$

So $V_{V-A B C D}=\frac{1}{3} \cdot 15^{2} \cdot 10=750$.