input: Write a function to find the common elements in two lists. Return the common elements in a new list.
  find_common_elements(list1, list2):
  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([10, 20, 30], [5, 15, 25])==[]
  assert find_common_elements([], [1, 2, 3])==[]
output: find_common_elements(list1, list2):
      return list(set(list1) & set(list2))