KarlKnechtel: threatens at which would make the second eye false; White must block. After , White can't connect with at because of shortage of liberties; and with any other response captures, preventing the requisite two eyes. Even with the capture and connection of , still destroys the eye, so White is dead.
The shortage of liberties threat which follows shortly after at a is the only key to solve this one. If Black omits the exchange of a for b White is alive after .
Moosky: The circled spot becomes an eye if W wins this ko fight, thus this solution fails. (B can push by playing at , but W will connect at to secure the eye at ).
If Black tries this , White captures at a, making x and y miai to ensure the eye at .