Forbidden Alchemy

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.

Wotc — March 19, 2021

"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."

Wotc — March 19, 2021

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.

Wotc — March 19, 2021

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.

Wotc — March 19, 2021

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

Wotc — March 19, 2021

You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.

Wotc — March 19, 2021

If you have fewer than four cards in your library, you'll look at all the cards there and put one into your hand and the rest into your graveyard.

Wotc — September 22, 2011