a) $$(a-4)(a-4) = a^2 - 4a - 4a + 16 = a^2 - 8a + 16$$
б) $$(3x+5)(4x-2) = 12x^2 - 6x + 20x - 10 = 12x^2 + 14x - 10$$
в) $$(2a+3b)(4a+5b) = 8a^2 + 10ab + 12ab + 15b^2 = 8a^2 + 22ab + 15b^2$$
г) $$(x-3)(x^2+3x+4) = x^3 + 3x^2 + 4x - 3x^2 - 9x - 12 = x^3 - 5x - 12$$