(a-2b-3c)^2
=(a-2b)^2-6c(a-2b)+9c^2
=(a-2b)^2-6ac+12bc+9c^2
=a^2-4ab+b^2-6ac+12bc+9c^2
（2y-1)(4y^2+1)(2y+1)
=（2y-1)(2y+1)(4y^2+1)
=（4y^2-1)(4y^2+1)
=16y^2-1