Return the Past

If you cast an instant or sorcery with X generic mana in its mana cost this way, you still choose the value of X as part of casting the spell and pay that cost.

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

If a card is put into your graveyard during your turn, you can cast it with flashback if it's legal to do so before any other player can take any actions.