Consider the function $f(x) = 3 x + 6 x ^ { -1 }$.
Note that this function has no inflection points, but $f''(x)$ is undefined at $x=B$ where
$B=$
For each of the following intervals, tell whether $f(x)$ is concave up (type in CU) or concave down (type in CD).
$(-\infty, B)$:
$(B,\infty)$: