(x^2+3x)^2-2x^2-6x-8
=(x^2+3x)^2-2(x^2-3x)+1-9
=(x^2+3x-1)^2-3^2
=(x^2+3x+2)*(x^2+3x-4)
=(x-1)*(x+1)*(x+2)*(x+4)