(l+m)x^3+(3l+2m-n)x^2+(2l-m-3n)x-2(m+n)
=(l+m)x^3+(l+m)x^2+(2l+m-n)x^2+(2l-m-3n)x-2(m+n)
=(l+m)x^2(x+1)+[(2l+m-n)x-2(m+n)](x+1)
=(x+1)[(l+m)x^2 +(2l+m-n)x-2(m+n)]
=(x+1)[(l+m)x-(m+n)](x+2)