This is a go classics problem. I would like you to think about it a little. Then we will go through the variants, not just the correct one, because it is important to know what to do if your opponent makes a mistake, right? When you are ready, scroll down.
As you can see, White has only one eye here. on the other hand, Black must not play like this:
Black 18 at 11.
The correct way to play for White:
This is the correct move. How can Black respond? - in case he thinks he can kill White of course...
Obviously, this is not the best way. White connects underneath and escapes.
Yes. This is the only way. But what now? The position is identic for both players, Black has the advantage of a move and White is faced to the corner. But wait! Black has to respond here. His stones (the cut out ones) are also in danger!
It is important for White now not to get dragged in the capture - the marked stones should be his target, not the two ones. If he decides to continue the capture of the two stones, he will lose liberties and will be captured himself.
The move at 6 is very important. White cannot live in the corner. If Black plays a bit more differently, the sequence results as shown below:
And Black is captured. Nevertheless, this is not the solution, as Black has a good move at 6 in the diagram above. Let's see another possibility:
White 1 aims at the three marked stones and threatens to make two eyes after playing at X and then 2. Black then prevents White from doing that. Shall White start the capture now?
As you can see, Black can escape here. White has to do something about the two marked stones. Fortunately, there is a good stone to use as a forcing move support:
The rest should be obvious to anyone playing go more than a month.
Black 4 at the marked stone.