Indicating the onset of instability on a graph

[Pietair]

Good day,

Homework Statement

Indicate on the graph the onset of instability.

The graph:

I really have no idea how I can indicate the onset of instability on such a graph.

Thanks in advance!

 

[KLoux]

Why do you think your system is unstable?

 

[Pietair]

It isn't, that's the reason I really don't know how to indicate the onset of instability on that graph...

 

[KLoux]

Well then there's your answer... If it's not unstable, how can you indicate the point where it becomes unstable?