21 : (x - 7) = 7

Решение:

1. \( 21 : (x - 7) = 7 \)
2. \( 21 = 7 \cdot (x - 7) \)
3. \( \frac{21}{7} = x - 7 \)
4. \( 3 = x - 7 \)
5. \( x = 3 + 7 \)
6. \( x = 10 \)

Ответ: 10