At the beginning of your combat phase, Iroh triggers, giving Hellkite Charger firebending 2 until end of turn.
Declare Hellkite Charger as an attacker, triggering its innate ability and each instance of firebending it has.
Resolve all firebending triggers, adding for each trigger.
Resolve the other Hellkite Charger trigger, causing you to pay 5 to untap all attacking creatures you control and get an additional combat phase after this one.