\(x^2-x-12=0\)
\(\Rightarrow x^2+3x-4x-12=0\)
\(\Rightarrow x\left(x+3\right)-4\left(x+3\right)=0\)
\(\Rightarrow\left(x-4\right)\left(x+3\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}x-4=0\\x+3=0\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=4\\x=-3\end{matrix}\right.\)
