一瞬の瞬き

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.
A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.
When the card returns to the battlefield, it will be a new object with no connection to the card that was exiled. Auras attached to the exiled creature will be put into their owners’ graveyards. Any Equipment will become unattached and remain on the battlefield. Any counters on the exiled creature will cease to exist.
“Flashback [cost]” means “You may cast this card from your graveyard 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.”
You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.
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.
If a token is exiled this way, it will cease to exist and won’t return to the battlefield.
If a card with flashback is put into your graveyard during your turn, you can cast it if it’s legal to do so before any other player can take any actions.