Non-Joseki Exercise 4 / Solution

Simplest  

Then B at a or b depending which direction is more important.

alternatives  

White might choose different moves instead of white 4, but none will do her much good. After white 4 here, black plays atari at 5, then presses at 7. If white cuts at a, black is willing to sacrifice the corner. If that is not to his liking, black b instead of 7 is an alternative.