input: Write a function to combine two strings and reverse the result in Python. The function should take two strings as input and return the reversed combined string.
  assert reverse_combined_strings("Hello ", "world!")=="!dlrow olleH"
  assert reverse_combined_strings("Python ", "Programming")=="gnimmargorP nohtyP"
  assert reverse_combined_strings("Open ", "AI")=="IA nepO"
  assert reverse_combined_strings("123 ", "456")=="654 321"
  assert reverse_combined_strings("", "AI")=="IA"
output: def reverse_combined_strings(str1, str2):
      return (str1 + str2)[::-1]