71/3 = (7 * 3 + 1) / 3 = 22/3
22/3 + 8/9 = (22 * 3) / (3 * 3) + 8/9 = 66/9 + 8/9 = 74/9
1/8 · 74/9 = 74 / (8 * 9) = 74/72
74/72 = 37/36
37/36 - 13/12 = 37/36 - (13 * 3) / (12 * 3) = 37/36 - 39/36 = -2/36
-2/36 = -1/18
Ответ: -1/18