input: Write a function to combine two strings and reverse the result in Python. The function should concatenate the two input strings and then return the reversed version of the concatenated result.
  assert combine_and_reverse("Hello ", "world!")=="!dlrow olleH"
  assert combine_and_reverse("Python ", "is awesome!")=="!emosewa si nohtyP"
  assert combine_and_reverse("Programming ", "is fun!")=="!nuf si gnimmargorP"
  assert combine_and_reverse("OpenAI ", "ChatGPT!")=="!TGpuC IAnepO"
  assert combine_and_reverse("", "")==""
  assert combine_and_reverse("a", "b")=="ba"
output: def combine_and_reverse(str1, str2):
      return (str1 + str2)[::-1]