Answer
See the answer below.
Work Step by Step
$\ \ \ \ HPO_4^{2-}+H_2O\leftrightarrow PO_4^{3-}+H_3O^+\ -->K_{a3}$
$+ \ PO_4^{3-}+H_2O\leftrightarrow HPO_4^{2-}+OH^-\ --> K_{b1}$
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$\ \ \ \ 2\ H_2O\leftrightarrow H_3O^++OH^-\ --> K_w$
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