input: Write a python function to retrieve strings of a given length from a specified list of strings.
  assert retrieve_strings(['Python', 'list', 'exercises', 'practice', 'solution'], 8) == ['practice', 'solution']
  assert retrieve_strings(['Python', 'list', 'exercises', 'practice', 'solution'], 6) == ['Python']
  assert retrieve_strings(['Python', 'list', 'exercises', 'practice', 'solution'], 9) == ['exercises']
output: def retrieve_strings(lst, length):
      return [s for s in lst if len(s) == length]