City on Fire

• WOTC Official Reminder Textofficial rules
  Convoke — Your creatures can help cast this spell. Each creature you tap while casting this spell pays for or one mana of that creature's color.

• WWizards of the Coast2023-04-14
  Official Ruling
  If you control two Cities on Fire, damage dealt by sources you control will be multiplied by 9. If you control three, it will be multiplied by 27, and so on.
• WWizards of the Coast2023-04-14
  Official Ruling
  If an effect such as that of Chandra's Pyrohelix asks you to divide damage among targets, you must divide the unmodified damage before tripling it.
• WWizards of the Coast2023-04-14
  Official Ruling
  While you control City on Fire, if a creature you control with trample would deal combat damage to a blocking creature, you must assign its unmodified damage. For example, a 3/3 creature with trample blocked by a 2/2 creature can have 1 damage assigned to the defending player. It will then deal 6 damage to the blocking creature (2 tripled) and 3 to the defending player (1 tripled).
• WWizards of the Coast2024-01-12
  Official Ruling
  Tapping a multicolored creature using convoke will pay for or one mana of your choice of any of that creature's colors.
• WWizards of the Coast2024-01-12
  Official Ruling
  When calculating a spell's total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke applies after the total cost is calculated. Convoke doesn't change a spell's mana cost or mana value.
• WWizards of the Coast2024-01-12
  Official Ruling
  If a creature you control has a mana ability with in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell's costs. You won't be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won't be on the battlefield when you pay the spell's costs, so you won't be able to tap it for convoke.
• WWizards of the Coast2024-01-12
  Official Ruling
  Tapping an untapped creature that's attacking or blocking to convoke a spell won't cause that creature to stop attacking or blocking.
• WWizards of the Coast2024-01-12
  Official Ruling
  You can tap any untapped creature you control to convoke a spell, even one you haven't controlled continuously since the beginning of your most recent turn.
• WWizards of the Coast2024-01-12
  Official Ruling
  Because convoke isn't an alternative cost, it can be used in conjunction with alternative costs.

Rulings via Scryfall