\(1,\)
\(x^2+x-12\)
\(=x^2-3x+4x-12\)
\(=x\left(x-3\right)+4\left(x-3\right)\)
\(=\left(x+4\right)\left(x-3\right)\)
\(2,\)
\(x^2-9x+20\)
\(=x^2-4x-5x+20\)
\(=x\left(x-4\right)-5\left(x-4\right)\)
\(=\left(x-5\right)\left(x-4\right)\)
\(3,\)
\(x^2+x-20\)
\(=x^2-4x+5x-20\)
\(=x\left(x-4\right)+5\left(x-4\right)\)
\(=\left(x+5\right)\left(x-4\right)\)