Chandra, Pyromaster's ability and spells with x casting cost
X is always zero in this case according to this rule.


  107.3b If a player is casting a spell that has an {X} in its mana cost, the value of X isn’t defined by the text of that spell, and an effect lets that player cast that spell while paying neither its mana cost nor an alternative cost that includes X, then the only legal choice for X is 0.