Flashback

If a card with no mana cost gains flashback, it has no flashback cost. It can't be cast this way.

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

A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.

"Flashback [cost]" means "You may cast this card from your graveyard if the resulting spell is an instant or sorcery spell by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack."

If you cast a spell with flashback, you can't pay any alternative costs. You can pay additional costs such as kicker costs. If the spell has any mandatory additional costs, you must pay those to cast the spell with flashback.

You must still follow any timing restrictions and permissions, including those based on the card's type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.