Let's look at the reduction half reaction. Remember, the MNS are already balanced out, so let's balance out oxygen. I have four oxygen this side, so that means you have to add four water molecules on this side. If I add four water molecules on this side, I balance out the O. But now I have to balance out the H, because now I have four times two, eight H ions on this side or atoms on this side. That means I balance this guy out by adding eight H plus ions to this side, and I get this following reaction.