Whenever a creature with flying enters the battlefield under your control, it gains haste until end of turn.
Whenever a Dragon enters the battlefield under your control, it deals X damage to any target, where X is the number of Dragons you control.
11/17/2017 The amount of damage dealt by the Dragon that entered the battlefield is based on the number of Dragons you control when the ability resolves.
11/17/2017 If a Dragon entering the battlefield causes Dragon Tempest’s second ability to trigger but leaves the battlefield before that ability resolves, that Dragon still deals damage.
11/17/2017 The two abilities aren’t mutually exclusive. If a Dragon with flying (which is most of them) enters the battlefield under your control, both abilities will trigger.