a)
`x^2-z^2-6x+9`
`=x^2-6x+9-z^2`
`=(x-3)^2-z^2`
`=(x-3-z)(z-3+z)`
b)
`x^3+x^2-x-1`
`=x^3-x^2+2x^2-2x+x-1`
`=x^2(x-1)+2x(x-1)+(x-1)`
`=(x^2+2x+1)(x-1)`
`=(x+1)^2(x-1)`
c)
`x^3-4x^2+4x-1`
`=x^3-x^2-3x^2+3x+x-1`
`=x^2(x-1)-3x(x-1)+(x-1)`
`=(x^2-3x+1)(x-1)`