X^4-x^3-7x^2+x+6=0
x^4-x^3-x^2-6x^2+x+6=0
x^4-x^3-x^2+x-6x^2+6=0
x^3(x-1)-x(x-1)-6(x^2-1)=0
x^3(x-1)-x(x-1)-6(x-1)(x+1)=0
(x-1)(x^3-7x-6)=0
(x-1)(x^3-x-6x-6)=0
(x-1)(x(x^2-1)-6(x+1)=0
(x-1)(x+1)(x^2-x-6)=0
(x-1)(x+1)(x-3)(x+2)=0
откуда х=1, х=-1, х=3, х=-2