Simply remove #H^+# from your base from your base formula; conserve mass and charge.
Explanation:
The conjugate base of #H_3PO_4#, phosphoric acid, is #H_2PO_4^-#. The conjugate base of dihydrogen phosphate is #HPO_4^(2-)# (so you know the conjugate acid, why?).
You have #HPO_4^(2-)#, so its conjugate base is.......................???????