Solution by lavalyn:
In symmetrical positions, one useful proverb is to play on the point of symmetry. White cannot defend both snapbacks (without loss of generality the left side is defended) and thus the only real eye in the White group is the marked point - White dies a horrible death.
timh?: Is this another option for following up on ? Then white needs both a and b to survive, so black takes the other one to kill.
From my reading, while it does also do the job, above gives W 3 ko threats to live in the future. (3rd threat is under the stones at the marked W stone).
In the snapback solution, W only have 2 ko threat to live: the atari and the under-the-stones play. The creation of an eye at the top (at a or b) is no longer a threat to live.
Anonymous: But couldn't White live by playing at ? If Black follows up with a, White captures both stones at b, and if Black follows with b, White plays a and lives.
Reuven: is self atari, B will capture the 3 stones now. If W then plays at b, trying to make an eye -
will be played under
, leaving it as a fake one.