（x²+x+1）(x²+x+2）-12
=（x²+x+1）(x²+x+1）+（x²+x+1）-12
=(x²+x+1+4)(x²+x+1-3)
=(x²+x+5)(x²+x-2)
=(x²+x+5)(x+2)(x-1)