посчитайте УРАВНЕНИЕРаспишите

к = 5 24/25

11 7/15 - (к - 3 1/75) = 8 13/25

172/15 - (к - 226/75) = 213/25

к - 226/75 = 172/15 - 213/25

к - 226/75 = (172*5 - 213*3)/75

к - 226/75 = (860 - 639)/75

к - 226/75 = 221/75

к = 221/75 + 226/75

к = 447/75 = 5 24/25

Проверим:

11 7/15 - (5 24/25 - 3 1/75) = 8 13/25

11 35/75 - (5 72/75 - 3 1/75) = 8 13/25

11 35/75 - 2 71/75 = 8 13/25

10 110/75 - 2 71/75 = 8 13/25

8 39/75 = 8 13/25

8 13/25 = 8 13/25 - верно