At 298 K, a certain buffer solution contains equal concentrations of X^- and HX, K b for X^- is 10^-10. What is the pH of this buffer solution?

At $\displaystyle 298$ K, a certain buffer solution contains equal concentrations of $\displaystyle \mathrm{X}^{-}$ and HX, $\displaystyle \mathrm{K}_{\mathrm{b}}$ for $\displaystyle \mathrm{X}^{-}$ is $\displaystyle 10^{-10}$. What is the pH of this buffer solution ?