а) (5у + 3у²) - (у² - 4у) = 5у + 3у² - у² + 4у = 2у² + 9у

b) 4a × (3b + 4a) - (4a)² = 12ab + 16a² - 16a² = 12ab

c) (a +4)(a - 2) = a² - 2a + 4a - 8 = a² + 2a - 8