There are actually two solutions for this problem.
Toey: Regarding solution 2, what will happen if White instead plays at ? Can Black still make two eyes on top?
unkx80: Yes, by capturing three white stones.
But suppose White pushes up with first? That's suicidal -- Black just captures all the stones with a series of atari moves.. (I put this in for reference; I thought White could escape, until I made up the diagram. -- Scartol)
does not expand his eye space enough.