9. If the graph of the function $g(x)$ is symmetric to the graph of the function $f(x)=\frac{1-2^{x}}{1+2^{x}}$ with respect to the line $y=x$, then the value of $g\left(\frac{2}{7}\right)$ is $\qquad$ .
9. $\log _{2} \frac{5}{9} \cdot \frac{1-2^{x}}{1+2^{x}}=\frac{2}{7}$, we get $9 \cdot 2^{x}=5, x=\log _{2} \frac{5}{9}$.