Consider the function $f(x) = 3 x + 6 x ^ { -1 }$. For this function there are four important intervals: $(-\infty, A]$, $[A,B)$,$(B,C)$, and $[C,\infty)$ where $A$, $B$ and $C$ are either critical numbers or points at which $f(x)$ is undefined.
Find $A$
and $B$
and $C$