You may need more information later, but to start with:
Vulnerable at imps you force to game as South opposite a 1S opening. You play an artificial system where the auction goes
1S - 2H - 2S* - 2NT* - 3S* - 3NT.
North has shown a 5=2=3=3 12-14 balanced. South has shown nothing other than 5 hearts and a game force.
EW are a solid expert pair who mainly play high stake rubber bridge.
West leads the 6 of diamonds, 4th highest.
You play the 10 from dummy. East thinks a bit, then plays the 9.
You play a heart to the king (agree?) which holds. That doesn't automatically tell you where the ace is.
(You might have preferred to win the diamond in hand if you were certain the king was on your left, but I don't think you are)
What's your plan for 9 tricks?