(i) Required expression = (4 - 5x + 6x2 - 8x3) - (6x3 - 7x2 + 5x - 3)
= 4 - 5x + 6x2 - 8x3 - 6x3 + 7x2 - 5x + 3
= - 8x3- 6x3 + 7x2 + 6x2- 5x - 5x + 3 + 4
= - 14x3 + 13x2 - 10x +7
(ii) Required expression = (5x2 - y + z + 7) - (- x2 - 3z)
= 5x2 - y + z + 7 + x2 + 3z
= 5x2+ x2 - y + z + 3z + 7
= 6x2 - y + 4z + 7
(iii) Required expression = (y3 - 3xy2 - 4x2y) - (x3 + 2x2y + 6xy2 - y3)
= y3 - 3xy2 - 4x2y - x3 - 2x2y - 6xy2 + y3
= y3 + y3 - 3xy2- 6xy2 - 4x2y - 2x2y - x3
= 2y3- 9xy2 - 6x2y - x3