Appa, Loyal Sky Bison

Since you are using an alternative cost to cast a spell from exile this way, you can't pay any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.

If the spell you cast has X generic mana in its mana cost, you must choose 0 as the value of X.

To airbend one or more permanents and/or spells means to exile each of those objects. For each card exiled this way, for as long as it's exiled, its owner may cast it from exile by paying 2 generic mana rather than paying its mana cost.