7 H2SO4 + K2Cr2VIO7 + 3 SnIISO4 7 H2O + K2SO4 + Cr2III(SO4)3 + 3 SnIV(SO4)2

This is an oxidation-reduction (redox) reaction:

3 SnII - 6 e- 3 SnIV (oxidation)

2 CrVI + 6 e- 2 CrIII (reduction)

Change the oxidation states