$y=-3x-12$
Subtracting is the same as adding the opposite$y=-3x+\left( -12 \right)$
Since the equation is written in slope-intercept form, $y=mx+b$, identify the slope of the line as the coefficient next to the variable $x$$\begin{array} { l }y=-3x+\left( -12 \right),& m=-3\end{array}$
Identify the $y$-intercept of the line as the constant term$\begin{array} { l }y=-3x+\left( -12 \right),& m=-3,& b=-12\end{array}$
The slope of the line is $m=-3$ and the $y$-intercept is $b=-12$$\begin{array} { l }m=-3,& b=-12\end{array}$