(x + 15) - 8 = 17
x + 15 = 17 + 8
x + 15 = 25
x = 25 - 15
x = 10
проверка
(10 + 15) - 8 = 17
25 - 8 = 17
17 = 17
(24 + x) - 21 = 10
24 + x = 10 + 21
24 + x = 31
x = 31 - 24
x =7
(24 + 7) - 21 = 10
31 - 21 = 10
10=10
(x + 15) - 8 = 17
x + 15 = 17 + 8
x + 15 = 25
x = 25 - 15
x = 10
проверка
(10 + 15) - 8 = 17
25 - 8 = 17
17 = 17
(24 + x) - 21 = 10
24 + x = 10 + 21
24 + x = 31
x = 31 - 24
x =7
проверка
(24 + 7) - 21 = 10
31 - 21 = 10
10=10