x^4+2*x^3+6x^2+5x
= x ( x^3+2x^2+6x+5)
= x [ (x^3+x^2) + (x^2+6x+5) ]
= x [ x^2 (x+1) + (x+1) (x+5 ]
= x (x+1) (x^2+x+5)