Factorize $18x^3 + 15x^2 - x - 2$:

The roots $x=\tfrac13, -\tfrac23, -\tfrac12$ each make the cubic vanish, corresponding to factors $(3x-1)$, $(3x+2)$, and $(2x+1)$. Their product reproduces $18x^3 + 15x^2 - x - 2$.