H2C2O4(aq) + HPO4^2- --> HC2O4^- + H2PO4-
The conjugate base of a weak acid is the anion which remains when H+ is removed.
H2C2O4 <==> H+ + HC2O4^-
acid ............. .......... conjugate base
H2PO4^- <==> H+ + HPO4^2-
acid ............. .......... conjugate base