Memory Deluge

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.

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

Memory Deluge cares about the mana you actually paid to cast it, not its mana cost. If you cast Memory Deluge for {2}{U}{U}, you will look at four cards. If you cast it with flashback for {5}{U}{U}, you will look at seven cards. Any effects that increase or decrease the cost to cast it will also be taken into account.

"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 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.

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