Ответ: 3.5

16x = 7(x + 4,5)

16x = 7x + 31.5

16x - 7x = 31.5

9x = 31.5

x = 31.5 : 9

x = 3.5

Проверка:

16 * 3.5 = 7 ( 3.5 + 4.5)

56 = 7 * 8

56 = 56