What is the conjugate base of HPO42-?

Simply remove ${H}^{+}$ from your base from your base formula; conserve mass and charge.
The conjugate base of ${H}_{3} P {O}_{4}$, phosphoric acid, is ${H}_{2} P {O}_{4}^{-}$. The conjugate base of dihydrogen phosphate is $H P {O}_{4}^{2 -}$ (so you know the conjugate acid, why?).
You have $H P {O}_{4}^{2 -}$, so its conjugate base is.......................???????