MgCe2+H3 PO4=

It looks like you might be asking about the reaction of magnesium with phosphoric acid. If so, the balanced equation is:

3 Mg + 2 H3PO4 → Mg3(PO4)2 + 3 H2

Explanation: Mg is oxidized from 0 to +2, phosphate stays as PO4^3− in Mg3(PO4)2, and hydrogen gas is produced to balance H.

If you meant something else (for example a compound like MgCe2(PO4)3 or involving Ce2+ specifically), please clarify the intended species and I’ll help balance that equation.