4) $$(6x^2 + 3x + 8) - (4x^2 - 2x + 5) = 6x^2 + 3x + 8 - 4x^2 + 2x - 5 = (6x^2 - 4x^2) + (3x + 2x) + (8 - 5) = 2x^2 + 5x + 3$$
Ответ: $$2x^2 + 5x + 3$$