input: Write a function to combine two strings and return the reversed result. The function should take two input strings, concatenate them, and then reverse the concatenated result.
  assert combine_and_reverse("Hello ", "world!")=="!dlrow olleH"
  assert combine_and_reverse("Python ", "Programming")=="gnimmargorP nohtyP"
  assert combine_and_reverse("OpenAI ", "GPT-4")=="4-TPG IAnepO"
  assert combine_and_reverse("", "empty")=="ypme"
  assert combine_and_reverse("short ", "")=="trohs"
output: def combine_and_reverse(s1, s2):
      return (s1 + s2)[::-1]