Suppose that $$f(x) = 3 x^3 + 6 x.$$

(A) Find all critical values of $f$. If there are no critical values, enter -1000. If there are more than one, enter them separated by commas.
Critical value(s) =

(B) Use interval notation to indicate where $f(x)$ is increasing.

Note: When using interval notation in WeBWorK, you use I for $\infty$, -I for $-\infty$, and U for the union symbol. If there are no values that satisfy the required condition, then enter (0,0) .

Increasing:

(C) Use interval notation to indicate where $f(x)$ is decreasing.
Decreasing:

(D) Use interval notation to indicate where $f(x)$ is concave up.
Concave up:

(E) Use interval notation to indicate where $f(x)$ is concave down.
Concave down:

(F) Find all inflection points of $f$. If there are no inflection points, enter -1000. If there are more than one, enter them separated by commas.
Inflection point(s) at $x$ =