Let $f(x)=2x^3-3x^2-39x6.$

$($a$)$ Find the points on the graph of $f$ at which the slope of the tangent line is equal to $-3.($If an answer does not exist, enter DNE.$)$

smaller $x-$value $$ x$)$

larger $x-$value $([,x])$

$($b$)$ Find the equation$($s$)$ of the tangent line$($s$)$ of part $($a$).($Let $x$ be the independent variable and $y$ be the dependent variable. If an answer does not exist. enter DNE.$)$

smaller $x-$value

$y=q,x$

larger $x-$value $y=$