x^3+6x^2y-xy^2-30y^3
=x^3+6x^2y-16xy^2+15xy^2-30y^3
=x(x^2+6xy-16y^2)+15xy^2-30y^3
=x(x-2y)(x+8y)+15y^2(x-2y)
==(x-2y)(x^2+8xy+15y^2)
=(x-2y)(x+3y)(x+5y)