Write a function `extract_string(strings, length)` that takes a list of strings and an integer `length` as input and returns a list of strings from the input list that have exactly the specified length.
#### Example Usage
```python [main.nopy]
extract_string(['Python', 'list', 'exercises', 'practice', 'solution'], 8)
# Output: ['practice', 'solution']
extract_string(['Python', 'list', 'exercises', 'practice', 'solution'], 6)
# Output: ['Python']
extract_string(['Python', 'list', 'exercises', 'practice', 'solution'], 9)
# Output: ['exercises']
```