input: Write a python function to find common elements between two lists.
  assert common_elements([1, 2, 3, 4], [3, 4, 5, 6]) == [3, 4]
  assert common_elements([10, 20, 30], [20, 40, 60]) == [20]
  assert common_elements([7, 8, 9], [10, 11, 12]) == []
output: def common_elements(list1, list2):
      return list(set(list1) & set(list2))