The unbalanced equation is
K2Cr2O7+FeSO4+H2SO4→K2SO4+Cr2(SO4)3+Fe2(SO4)3+H2O.
Balance all atoms except H and O.
K2Cr2O7+2FeSO4+5H2SO4→K2SO4+Cr2(SO4)3+Fe2(SO4)3+H2O.
The oxidation number of Cr decreases from +6 to +3. The decrease in oxidation number of 1 Cr atom is 3. The decrease in oxidation number of 2 Cr atoms is 6.
The oxidation number of Fe increases from +2 to +3. The increase in oxidation number of 1 Fe atom is 1. The increase in oxidation number of 2 Fe atom is 2.
Multiply species containing Fe with 3. It will balance increase in oxidation number with decrease in oxidation number.
K2Cr2O7+6FeSO4+7H2SO4→K2SO4+Cr2(SO4)3+3Fe2(SO4)3+H2O.
Balance O atoms by adding water molecules.
K2Cr2O7+6FeSO4+7H2SO4→K2SO4+Cr2(SO4)3+3Fe2(SO4)3+7H2O.
H atoms are balanced. This is balanced chemical equation.