you rebound a card with x in its casting cost ?
From the rulings on {Cast through time}.
6/15/2010 If you cast a card from exile "without paying its mana cost," you can't pay any alternative costs. Any X in the mana cost will be 0. On the other hand, if the card has optional additional costs (such as kicker or multikicker), you may pay those when you cast the card. If the card has mandatory additional costs (such as Momentous Fall does), you must pay those if you choose to cast the card.