Cr2O72-+ 14H+ + 6e- → 2Cr3+ + 7H2O
This one’s a little tricky…
The overall charge of the reactants is +12, because of the 14H+ and the one Cr2O72-
The overall charge of the products is +6 because of the 2Cr3+
Therefore, we add 6 electrons to the reactants...