9 * (5х + 13) + 12 = 264
9 * (5х + 13) = 264 - 12
9 * (5х + 13) = 252
5х + 13 = 252 : 9
5х + 13 = 28
5х = 28 - 13
5х = 15
х = 15 : 5
х = 3
ПРОВЕРКА:
9 * (5 * 3 + 13) + 12 = 264
9 * (15 + 13) + 12 = 264
9 * 28 + 12 =264
252 + 12 = 264
264 = 264
ОТВЕТ: х = 3