We need to consider all suit distributions: 5-0, 4-1 and 3-2 (both ways round).  Start by cashing the king.

If west has 5 hearts you have two marked finesses.  Come back to hand via the ace and lead low towards dummy.  You will eventually win the jack, as well as the ace and king.

If east has 5 hearts, again you have two marked finesses.  Lead low from dummy and just cover anything east plays.  You will lose to the queen or the 10 but your 8/9 combination will give you your third trick.

If both follow to the first trick your second play should be low towards hand.  Then if east shows out you take the ace, and lead towards the J-9.  Again three tricks in the bag.

But if east follows, just cover whatever he plays (with either the ace or the 8). If west also follows, then hey presto, the suit breaks 3-2 and your fourth heart takes the required third trick.

Of course, west may show out.  But then we come back to the 8/9 combination which will always be good for the third trick.