Ответ:

1. 7х + 7у / 21; 7 = ху / 3

2. 8m - 4n / 4m; 4 = 2m - n / m

3. 9a + 12b / 3; 3 = 3a + 4b

4. 45c + 18d / 27d; 9 = 5c + 2d / 3d

5. 5x - xy / x; x = 5 - y

6. 6mn - 4m / 2m; 2m = 3n - 2m