3x+1 | 6x^4+2x^3-6x^2-14x-1 | 2x^3-2x-4
...........6x^4+2x^3
............................-6x^2-2x
.....................................-12x
.....................................-12x-4
..............................................3
3x+1 is not a factor because there is a remainder of 3 after the long division.