Because of 6x? +x？ - 1

=6x^3-3x^2+4x^2- 1

=(6x^3-3x^2)+(4x^2- 1)

=3x^2(2x- 1)+(2x+ 1)(2x- 1)

=(2x- 1)(3x^2+2x+ 1)

What about polynomial 6x? +x？ -1 is divisible by the polynomial 2x- 1.