input: Write a python function to concatenate two lists using the + operator and the extend() method.
  assert concatenate_lists([1, 2, 3], [4, 5, 6]) == ([1, 2, 3, 4, 5, 6], [1, 2, 3, 4, 5, 6])
  assert concatenate_lists(['a', 'b'], ['c', 'd']) == (['a', 'b', 'c', 'd'], ['a', 'b', 'c', 'd'])
  assert concatenate_lists([], [1, 2, 3]) == ([1, 2, 3], [1, 2, 3])
output: def concatenate_lists(list1, list2):
      concatenated_plus = list1 + list2
      list1_copy = list1[:]
      list1_copy.extend(list2)
      return concatenated_plus, list1_copy