MTG Graveyard casting with 'X' costs
X will be zero


  When Goblin Dark-Dwellers enters the battlefield, you may cast target instant or sorcery card with converted mana cost 3 or less from your graveyard without paying its mana cost. If that card would be put into your graveyard this turn, exile it instead.


You are casting it from your graveyard, but you don't pay its mana cost. In that case, zero must be chosen for X.

from Goblin Dark-Dwellers on Gatherer


  1/22/2016: If the instant or sorcery card has {X} in its mana cost, you must choose 0 as its value.