input: Write a function to find the common elements in two lists and return a list of those elements. If there are no common elements, return an empty list.
  assert find_common_elements([3, 9, 15, 21, 27], [21, 7, 13, 29, 31])==[21]
  assert find_common_elements([1, 2, 3], [4, 5, 6])==[]
  assert find_common_elements([7, 8, 9], [9, 8, 7])==[7, 8, 9]
  assert find_common_elements([], [1, 2, 3])==[]
  assert find_common_elements([], [])==[]
output: def find_common_elements(list1, list2):
      return list(set(list1) & set(list2))