`a)` `3x(4x-3)-(2x-1)(6x+5)`
`=12x^2-9x-(12x^2+10x-6x-5)`
`=12x^2-9x-(12x^2+4x-5)`
`=12x^2-9x-12x^2-4x+5`
`=-13x+5`
`b)` `3x(x-1)^2-2x(x+3)(x-3)+4x(x-4)`
`=3x(x^2-2x+1)-2x(x^2-9)+4x^2-16x`
`=3x^3-6x^2+3x-2x^3+18x+4x^2-16x`
`=x^3-2x^2+5x`
`c)` `(x-1)^3-(x+2)(x^2-2x+4)+3(x+4)(x-4)`
`=x^3-3x^2+3x-1-(x^3+8)+3(x^2-16)`
`=x^3-3x^2+3x-1-(x^3+8)+3x^2-48)`
`=x^3+3x-1-x^3-8-45`
`=3x-57`
`d)` `(x+1)(x^2+x+1)(x-1)(x^2-x+1)`
`=(x^2-1)(x^2+x+1)(x^2-x+1)`
`=(x^2-1)` ` [ (x^2+1)^2-x^2] `
`=(x^2-1)(x^4+2x^2+1-x^2)`
`=(x^2-1)(x^4+x^2+1)`
`=x^6-1`