Past in Flames

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

If a split card gains flashback, you pay only the cost of the half you're casting.

"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."

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.

Past in Flames affects only cards in your graveyard at the time it resolves. Instant and sorcery cards put into your graveyard later in the turn won't gain flashback.