Автор - zagipasapakhova
21 + ( 45 - х ) * 3 = 93
21 + 45 * 3 - х *3 = 93
21 + 135 - 3х = 93
156 - 3х = 93
3х = 156 - 93
3х = 63
х = 63 : 3
х = 21
проверка:
21 + ( 45 - 21 ) * 3 = 93
21 + 24 * 3 = 93
21 + 72 = 93
93 = 93