input: Write a python function to find common elements between two lists.
  assert common_elements([1, 2, 3, 4], [2, 4, 6, 8]) == [2, 4]
  assert common_elements([10, 20, 30], [30, 40, 50]) == [30]
  assert common_elements([5, 7, 9], [1, 3, 5, 7]) == [5, 7]
output: def common_elements(list1, list2):
      return list(set(list1) & set(list2))