The solution is , making miai of a and b.
Specifically, if descends , then and lives.
Be careful, if is greedy and plays at a, then White b removes the second eye.
If makes a placement, then and captures the stone and lives.
here is greedy. The placement at makes a and b miai for White.
The descent of is also a mistake. After the hane-connect of and , Black does not have enough space to form a second eye at the bottom.