input: Write a function to combine 2 strings and reverse the resulting string in Python. The function should take two parameters which are the strings to be combined, combine them, reverse the combined string, and return the reversed string.
  assert combine_and_reverse("Hello ", "world!")=="!dlrow olleH"
  assert combine_and_reverse("Python ", "programming")=="gnimmargorp nohtyP"
  assert combine_and_reverse("", "test")=="tset"
  assert combine_and_reverse("a", "b")=="ba"
  assert combine_and_reverse("123", "456")=="654321"
output: def combine_and_reverse(s1, s2):
      combined = s1 + s2
      return combined[::-1]