`a, A(x) = 6x^3 - x^2 + 5x - 2x^3 + 6 - 9x`
`= (6x^3 - 2x^3) - x^2 + (5x - 9x) + 6`
`= 4x^3 - x^2 - 4x + 6`
`B(x) = -x + 2x^3 - 4x^2 + 3 - 2x^2`
`= 2x^3 - (4x^2 + 2x^2) - x + 3`
`= 2x^3 - 6x^2 - x + 3`
`b, A(x) - B(x) = (4x^3 - x^2 - 4x + 6) - (2x^3 - 6x^2 - x + 3)`
`= 4x^3 - x^2 - 4x + 6 - 2x^3 + 6x^2 + x - 3`
`= (4x^3 - 2x^3) + (-x^2 + 6x^2) - (4x - x) + (6 - 3)`
`= 2x^3 + 5x^2 - 3x + 3`