input: Write a function to find the common elements in two lists. The function should return a list of common elements.
  assert find_common_elements([3, 9, 15, 21, 27], [21, 7, 13, 29, 31])==[21]
  assert find_common_elements([1, 2, 3, 4, 5], [4, 5, 6, 7, 8])==[4, 5]
  assert find_common_elements([], [1, 2, 3])==[]
  assert find_common_elements([1, 2, 3], [])==[]
  assert find_common_elements([1, 2, 3], [4, 5, 6])==[]
output: def find_common_elements(list1, list2):
      return list(set(list1) & set(list2))