Dieter: I take advantage of the occasion to see how I approach a tsumego:
1) Look for the VitalPoint
If Black gets a stone here in sente, he will make two eyes at the circled points.
2) Look for a pattern that works, opponent's help included
3) Investigate first move
So, we get White 2 from the "first pattern". If White 4 at 5, Black 5 at 4 lives. White 6 fills at the marked spot. Next, Black squeezes and lives with 11 at a. This I would call the main line.
3) Investigate other possibilities to see if there is any better and to appreciate why they fail:
Three other candidates: lure out the cutting stone at a, the awkward but not self evident b and playing the marked stone immediately.