Question

oxydation number of CrFe-3

Answer 1

The (II) and (III) are the oxidation states of the iron in the two compounds: +2 and +3 respectively. That tells you that they contain Fe2+and Fe3+ ions.

Answer 2

let oxidation state of Cr is x
then
x-3*2=0
x=+6

your answer is +6
ok