input: Write a function to combine two strings and return the reversed result. The function should take two input strings and concatenate them before reversing the concatenated string. Return the reversed string.
  assert reverse_combined_string("Hello ", "world!")=="!dlrow olleH"
  assert reverse_combined_string("Python", " code")=="edoc nohtyP"
  assert reverse_combined_string("", "Java")=="avaJ"
  assert reverse_combined_string("C++", "")=="++C"
  assert reverse_combined_string("", "")==""
output: def reverse_combined_string(s1, s2):
      return (s1 + s2)[::-1]