б) (3x+10y)- (6x + 3y) + (6y-8x);
$$3x + 10y - 6x - 3y + 6y - 8x = (3x - 6x - 8x) + (10y - 3y + 6y) = -11x + 13y$$
Ответ: $$-11x + 13y$$