Appa, Steadfast Guardian

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

Lands exiled this way cannot be played from exile. (Lands that were animated by earthbend will be returned to the battlefield tapped when they're exiled.)

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.

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 rather than paying its mana cost.

Tokens exiled this way will cease to exist and cannot be cast.