Assign an oxidation number to each element in the reaction.

CaF2(s) + H2SO4(aq) → CaSO4(aq) + 2HF(g)

In CaF2, the oxidation number of Ca is
, and that of F is
. In H2SO4, the oxidation number of H is
, that of S is
, and that of O is
. In CaSO4, the oxidation number of Ca is
, that of S is
, and that of O is
. In HF, the oxidation number of H is
, and that of F is
.