7. Let $f(x)=\lg \left(\sqrt{1+\pi^{2} x^{2}}-\pi x\right)+\pi$. If $f(m)=3$, then $f(-m)=$ $\qquad$
$$
\text { II. 7. } 2 \pi-3 \text {. }
$$

Given that $y=\lg \left(\sqrt{1+\pi^{2} x^{2}}-\pi x\right)$ is an odd function
$$
\begin{array}{l}
\Rightarrow f(m)+f(-m)=2 \pi \\
\Rightarrow f(-m)=2 \pi-3 .
\end{array}
$$