input: Write a function to combine two strings and reverse the result. The function should take two string inputs and return the reversed combined string.
  assert combine_and_reverse("Hello ", "world!")=="!dlrow olleH"
  assert combine_and_reverse("Python ", "is fun!")=="!nuf si nohtyP"
  assert combine_and_reverse("OpenAI ", "GPT-3")=="3-TPG IAnepO"
  assert combine_and_reverse("", "test")=="tset"
  assert combine_and_reverse(" ", "")==" "
output: def combine_and_reverse(str1, str2):
      return (str1 + str2)[::-1]