`c)`
`x^3+5x^2+3x-9`
`=x^3+6x^2+9x-x^2-6x-9`
`=x(x^2+6x+9)-(x^2+6x+9)`
`=(x-1)(x^2+6x+9)`
`=(x-1)(x+3)^2`
`d)`
`x^16+x^8-2`
`=x^16-x^8+2x^8-2`
`=x^8(x^8-1)+2(x^8-1)`
`=(x^8+2)(x^8-1)`
`=(x^8+2)[(x^4)^2-1]`
`=(x^8+2)(x^4-1)(x^4+1)`
`=(x^8+2)[(x^2)^2-1](x^4+1)`
`=(x^8+2)(x^2-1)(x^2+1)(x^4+1)`
`=(x^8+2)(x-1)(x+1)(x^2+1)(x^4+1)`