a) $$5 + 2x + x^2 - 3x = x^2 + (2 - 3)x + 5 = x^2 - x + 5$$.
б) $$3a^2 - 7a - 2a + 4 = 3a^2 + (-7 - 2)a + 4 = 3a^2 - 9a + 4$$.
в) $$x^3 - 2x + 3 - x^2 - 2x - 4 = x^3 - x^2 + (-2 - 2)x + (3 - 4) = x^3 - x^2 - 4x - 1$$.
г) $$4a^3 - 6a^2 - 3 - 6a^3 + 5a^2 = (4 - 6)a^3 + (-6 + 5)a^2 - 3 = -2a^3 - a^2 - 3$$.