Can i use defy on Shadow dash when its used from the trash paying Flow costs?

Yes. Defy can counter Shadow Dash even when it's played from the trash via Flow.

Defy only checks the printed cost of the spell, not what was actually paid — Shadow Dash's printed cost (2 energy, 1 power) meets Defy's restriction, so the Flow cost of [5][A][A] is irrelevant (Rule 206). Playing a spell via Flow doesn't change its timing or permissions, so Shadow Dash sits on the Chain as a normal spell and is a legal Defy target (Rule 829.1.b.2).

One wrinkle: if Defy counters it, Shadow Dash won't return to the trash — the Flow delayed replacement banishes it instead, since it's leaving the Chain without its own execution instructing that (Rule 829.1.b.1, 425.1.a.1).