8ac-6ab + 3b³ - (5ac-5ab-b³) = 8ac - 6ab + 3b³ - 5ac + 5ab + b³ = (8ac - 5ac) + (-6ab + 5ab) + (3b³ + b³) = 3ac - ab + 4b³
Ответ: 3ac - ab + 4b³