For $f(x)=x^4-12x^{3}8$ find the following.

$($A$)f^{\text{'}}(x)$

$($B$)$ The slope of the graph of $f$ at $x=-1$

$($C$)$ The equation of the tangent line at $x=-1$

$($D$)$ The value$($s$)$ of $x$ where the tangent line is horizontal

$($A$)f^{\text{'}}(x)=$

$($B$)$ At $x=-1,$ the slope of the graph of $f$ is $.$

$($C$)$ At $x=-1,$ the equation of the tangent line is $y=.$

$($D$)$ The tangent line is horizontal at $x=.$

$($Use a comma to separate answers as needed.$)$