x^2+3x=6
x^2=-3x+6
x^3=x*x^2=x(-3x+6)=-3x^2+6x
所以
x^3+x^2+2x+20
=-3x^2+6x+x^2+2x+20
=-2x^2+8x+20
=-2(x^2-4x)+20
=-2*（-7x+6）+20
=14x-12+20
=14x+8