The first must be reversed (cool rhyme!)
When you "add together" two or more chemical equations, it goes similarly as algebra: equal quantities on opposite sides of either reaction can cancel out.
Here's what I mean by this:
Our two equations are
Here, there must be two duplicate quantities is a given color to cancel out. As you may see, neither red nor blue contains a duplicate species. However, if we reversed the first equation, we would then have
Two quantities can cancel out, leaving behind the blue equation: