City on Fire

Tapping a multicolored creature using convoke will pay for {1} or one mana of your choice of any of that creature's colors.

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.

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.

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).

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.