MgI₂(aq) + Ag₂CrO₄(s) → I₂(s) + Ag(s) + Mg²⁺(aq) + CrO₄^2–(aq)

basic environment

MgI₂(aq) → I₂(s) + Mg²⁺(aq) + 2 e^–

Ag₂CrO₄(s) + 2 e^– → 2 Ag(s) + CrO₄^2–(aq)

sum and eliminate common species

MgI₂(aq) + Ag₂CrO₄(s) → I₂(s) + 2 Ag(s) + Mg²⁺(aq) + CrO₄^2–(aq)

No H⁺(aq) to worry about! This could have been done by inspection easily.