Poker Quiz: Call or Fold with Nut Flush Draw?

The Situation:We’re in a loose-aggressive 6-max $50NL online cash game. Everyone has ~$50 stacks. We are in late position with A♠-K♠. A loose- aggressive player in the under-the-gun (UTG) seat open-raises to $2. We re- raise to $6, and the villain calls. The flop is 3♠-4♠-9♦. The villain checks and we bet $9 into the pot. The villain check-raises us all-in....