x的四次方-6x²-27
=(x²+3)(x²-9)
=(x²+2)(x+3)(x-3)
x的3次方-4x²-21x
=x(x²-4x-21)
=(x+3)(x-7)
x的四次方-7x²+6
=(x²-6)(x²-1)
=(x²-6)(x+1)(x-1)
如果本题有什么不明白可以追问,