I am a student of Economics and we just have very basic maths in our course. I am trying to study Quasi-convexity and quasi-concavity from this book called Alpha Chiang but it is not given very clearly in that. (Probably I am not able to understand it).

So, I went to Wikipedia to go through some definitions. I understand the condition mentioned on wikipedia page but I fail to undertand how the following function is NOT quasi convex.

Any comments / guidance in this regard will be highly appreciated.