(3/4 + 6/7) * (14/21 * 15/18) = (3 * 7 + 6 * 4)/(4 * 7) * (2/3 * 5/6) = (21 + 24)/28 * 5/9 = (45 * 5)/(28 * 9) = 25/28

Ответ: 25/28.