Breaching Dragonstorm enters the battlefield, its first ability triggers, causing you to exile the Dragon from the top of your library and cast it without paying its mana cost.
When the Dragon enters the battlefield,
Breaching Dragonstorm's second ability and
Mana Echoes trigger.
Resolve the
Breaching Dragonstorm trigger, returning it from the battlefield to your hand.
Resolve the
Mana Echoes trigger, adding ({C} magic symbol) for each Dragon you control.