Find the absolute maximum and minimum values of $f(x) = x^{2}-3x^{3}-7x+6$, if any, over the interval $(-\infty,+\infty)$.

absolute maximum is and it occurs at x =
absolute minimum is and it occurs at x =

Notes: If there is more than one $\small x$ value, enter as a comma separated list. Enter "none" in any unused answer box.