input: Write a python function to calculate the target sum by subtracting each element of the list from the given subtrahend.
  assert target_sum(10, [1, 2, 3, 4, 5]) == [9, 8, 7, 6, 5]
  assert target_sum(5, [2, 4, 6]) == [3, 1, -1]
  assert target_sum(0, [1, -1, 2, -2]) == [-1, 1, -2, 2]
output: def target_sum(subtrahend, lst):
      return [subtrahend - x for x in lst]