$$3a^2b - (-2a^3b^4) = 3a^2b + 2a^3b^4$$
Ответ: $$3a^2b + 2a^3b^4$$
$$(-3a^3b^2)^3 = (-3)^3 (a^3)^3 (b^2)^3 = -27a^9b^6$$
Ответ: $$-27a^9b^6$$