`b,(a+b)^3 - (a-b)^3 - 6a^2 b`
`=(a^3+3a^2 b+3ab^2 +b^3)-(a^3-3a^2 b+3ab^2 -b^3)-6a^2 b`
`=a^3+3a^2 b+3ab^2 +b^3-a^3 +3a^2 b-3ab^2 +b^3 -6a^2 b`
`=2b^3`
`c, (x^2 -1 )^3 - (x^4 + x^2+1)(x^2 -1)`
`=[(x^2)^3 -3(x^2)^2 .1 + 3x^2 .1^2 -1^3)-[(x^2)^3 -1^3]`
`=(x^6 -3x^4 + 3x^2 -1)-(x^6 -1)`
`=x^6 -3x^4 + 3x^2 -1-x^6 +1`
`= -3x^4 + 3x^2 `

