$pH=10/7\Rightarrow \left[ {{H}^{+}} \right]={{10}^{-10/7}}\Rightarrow \left[ {{H}^{+}} \right]\left[ O{{H}^{-}} \right]={{10}^{-14}}\Rightarrow \left[ O{{H}^{-}} \right]=\frac{{{10}^{-14}}}{{{10}^{-10/7}}}={{10}^{-3/3}}$ ${{K}_{b}}=\frac{{{\left[ O{{H}^{-}} \right]}^{2}}}{M-\left[ O{{H}^{-}} \right]}\Rightarrow \frac{{{({{10}^{-3/3}})}^{2}}}{0/1-{{10}^{-3/3}}}=\frac{{{({{10}^{-3/3}})}^{2}}}{0/1}=\frac{{{10}^{-6/6}}}{{{10}^{-1}}}={{10}^{-5/6}}$ ${{K}_{b}}={{10}^{-5}}\times {{10}^{-0/6}}={{10}^{-5}}\times {{({{10}^{-0/3}})}^{2}}={{10}^{-5}}\times {{(\frac{1}{2})}^{2}}=0/25\times {{10}^{-5}}=2/5\times {{10}^{-6}}$