If a card in a player's hand has in its mana cost, X is considered to be 0.
2018-12-07
If a spell has in its mana cost, include the value chosen for that X when determining the mana value of that spell, even if it was cast for an alternative cost and no mana was spent on X.
You may exile a green card with mana value X from your hand rather than pay this spell's mana cost.
in its mana cost, X is considered to be 0.