Cr2III(SO4)3 + 3 Fe2III(SO4)3 + K2SO4 + 7 H2O K2Cr2VIO7 + 6 FeIISO4 + 7 H2SO4

This is an oxidation-reduction (redox) reaction:

2 CrIII - 6 e- 2 CrVI (oxidation)

6 FeIII + 6 e- 6 FeII (reduction)

Change the oxidation states