Выполните действие: a) (2a - 1) / 3a + (a + 2) / 6a; б) 1 / (x - 3) - 1 / (x + 3).

a) $$\frac{2a - 1}{3a} + \frac{a + 2}{6a} = \frac{2(2a - 1) + (a + 2)}{6a} = \frac{4a - 2 + a + 2}{6a} = \frac{5a}{6a} = \frac{5}{6}$$ б) $$\frac{1}{x - 3} - \frac{1}{x + 3} = \frac{(x + 3) - (x - 3)}{(x - 3)(x + 3)} = \frac{x + 3 - x + 3}{x^2 - 9} = \frac{6}{x^2 - 9}$$