Groups and the Group Axioms · Hard problem

The table below is supposed to be the addition table of $\mathbb Z_6$ ($*$ entry in row $i$, column $j$, equal to $(i+j)\bmod6$), but two entries have been corrupted, each creating a repeated value in its row:

Find the sum of the two repeated values (the one in row $2$ plus the one in row $4$).