a) \(x^2-7x+12=x\left(x-4\right)-3\left(x-4\right)=\left(x-4\right)\left(x-3\right)\)
b) \(x^2-9x+20=x\left(x-5\right)-4\left(x-5\right)=\left(x-5\right)\left(x-4\right)\)
c) \(x^2-6x-7=x\left(x-7\right)+1\left(x-7\right)=\left(x-7\right)\left(x+1\right)\)
d) \(x^2-x-12=x\left(x-4\right)+3\left(x-4\right)=\left(x-4\right)\left(x+2\right)\)