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