Chapter 21, Problem 46QAP

Given
$\begin{array}{l}\text{HF}\rightleftharpoons {\text{H}}^{+}\left(aq\right)+{\text{F}}^{-}\left(aq\right)K_{\text{a}}\text{=}6.9\times {10}^{-4}\\ \text{HF}\left(aq\right)+{\text{F}}^{-}\left(aq\right)\rightleftharpoons {\text{H}}_2{}^{-}\left(aq\right)K\text{=2}\text{.7}\end{array}$
calculate K for the reaction
$\text{2HF}\left(aq\right)\rightleftharpoons {\text{H}}^{+}\left(aq\right)+{\text{HF}}_2{}^{-}\left(aq\right)$