`a)``(a^2+a-1)(a^2-a+1)`
`=a^4-a^3+a^2+a^3-a^2+a-a^2+a-1`
`=a^4-a^2+2a-1`
`b)``(a+2)(a-2)(a^2+2a+4)(a^2-2a+4)`
`=[(a+2)(a^2-2a+4)][(a-2)(a^2+2a+4)]`
`=(a^3+2^3)(a^3-2^3)`
`=a^6-2^6`
`=a^6-64`
`c)``(2+3y)^2-(2x-3y)^2-12xy`
`=2^2+12y+(3y)^2-(2x)^2+12xy-(3y)^3-12xy`
`=4+12y-4x^2`
`d)(x+1)^3-(x-1)^3-(x^3-1)-(x-1)(x^2+x+1)`
`=x^3+3x^2+3x+1-x^3+3x^2-3x+1-(x^3-1)-(x^3-1)`
`=6x^2+2-2(x^3-1)`
`=6x^2+2-2x^3+2`
`=-2x^3+6x^2+4`
