input: Write a function to combine two strings and return the reversed result. The function should take two strings as arguments, concatenate them, reverse the concatenated result, and return it. If one of the input strings is empty, it should return the reversed version of the non-empty string.
  assert reverse_combined_string("Hello ", "world!")=="!dlrow olleH"
  assert reverse_combined_string("abc", "")=="cba"
  assert reverse_combined_string("", "xyz")=="zyx"
  assert reverse_combined_string("", "")==""
output: def reverse_combined_string(s1, s2):
      combined = s1 + s2
      return combined[::-1]