3x^2-x=1
6x^3+7x^2-5x+2001
=6x^3-2x^2+9x^2-5x+2001
=2x(3x^2-x)+9x^2-5x+2001
=2x+9x^2-5x+2001
=9x^2-3x+2001
=3(3x^2-1)+2001
=3*1+2001
=2004