So, do you know what the notation means?
To translate:
f is a rival of g if there exists an x in the set of real numbers such that for all y in the set of real numbers, x being less than y implies that |f(x)-g(x)| is less than |f(y)-g(y)|
f is a frenemy of g if for all x in the set of real numbers, there exists some y in the set of real numbers such that x is less than y and |f(x)-g(x)| < |f(y)-g(y)|
So that being said, try looking at the problem again. I'll post again later if you still want more help.