Смотри, тут всё просто:
(45 ⋅ 63) : 81 = (5 ⋅ 9) ⋅ (7 ⋅ 9) : 81 = (35 ⋅ 81) : 81 = 35 ⋅ (81 : 81) = 35
Ответ: 7