input: Write a python function to return the sum of the 3rd and 7th elements in a list.
  assert sum_elements([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) == 10
  assert sum_elements([10, 20, 30, 40, 50, 60, 70, 80, 90]) == 100
  assert sum_elements([5, 15, 25, 35, 45, 55, 65, 75]) == 90
output: def sum_elements(lst):
      return lst[2] + lst[6]