(\frac{9a^2}{16b^2} - \frac{1}{4b}) : (3a - \frac{1}{4b}) = (\frac{3a}{4b} - \frac{1}{4b}) * (3a + \frac{1}{4b}) / (3a - \frac{1}{4b}) = (3a + \frac{1}{4b})
Подставляем a = 2/3 и b = -1/12:
3 * 2/3 + 1/(4 * (-1/12)) = 2 + 1/(-1/3) = 2 - 3 = -1
Ответ: -1