x^4-7x^2+6x
=x(x^3-7x+6)
=x[(x^3-1)-7x+7]
=x[(x-1)(x^2+x+1)-7(x-1)]
=x(x-1)(x^2+x+1-7)
=x(x-1)(x^2+x-6)
=x(x-1)(x+3)(x-2)