11. The function $f(x)$ satisfies $f(3+x)=f(3-x), x \in \mathbf{R}$, and the equation $f(x)=0$ has exactly three distinct real roots. The sum of these three real roots is $\qquad$
11. 9 .

Axis of symmetry $x=3, f(x)=0$ has three roots, so $f(3)=0$. By symmetry, the other two roots $x_{1}+x_{2}=6$, thus $x_{1}+x_{2}+x_{3}=9$.