input: Write a function to combine two strings and reverse the result in Python. Return the reversed combined string.
  assert reverse_combine_strings("Hello ", "world!")=="!dlrow olleH"
  assert reverse_combine_strings("Python ", "is fun!")=="!nuf si nohtyP"
  assert reverse_combine_strings("", "")==""
  assert reverse_combine_strings("123 ", "456")=="654 321"
  assert reverse_combine_strings("test", "")=="tset"
output: def reverse_combine_strings(s1, s2):
      return (s1 + s2)[::-1]