Answer:
-2
Step-by-step explanation:
z - 7 ≥ 0
z ≥ 7
| z - 7 | = z - 7, z ≥ 7
| z - 7 | = -z + 7, z < 7
z - 9 ≥ 0
z ≥ 9
| z - 9 | = z - 9, z ≥ 9
| z - 9 | = -z + 9, z < 9
| z - 7 | = -z + 7
| z - 9 | = -z + 9
|z - 7| - |z - 9|
-z + 7 - (-z + 9)
-z + 7 + z - 9
= -2
