Chandra's ability allows you to cast spell without paying its casting cost:
−7: Exile the top ten cards of your library. Choose an instant or sorcery card exiled this way and copy it three times. You may cast the copies without paying their mana costs.
(see http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=383204). What if I decide to cast a spell with an X casting cost, e.g. Fireball (see http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=376334)? What kind of damage will be dealt?