a) a - (a - (2a - 4)) = a - a + 2a - 4 = 2a - 4
Ответ: $$2a-4$$
б) 7x - ((y-x) + 3y) = 7x - (y - x + 3y) = 7x - (4y - x) = 7x - 4y + x = 8x - 4y
Ответ: $$8x-4y$$