input: Write a function to combine 2 strings and reverse the result in Python. The function should take two string inputs, concatenate them, reverse the concatenated string, and return the reversed string.
  assert reverse_combined_string("Hello ", "world!")=="!dlrow olleH"
  assert reverse_combined_string("Python", " is fun!")=="!nuf si nohtyP"
  assert reverse_combined_string("", "Test")=="tseT"
  assert reverse_combined_string("Case", "")=="esaC"
  assert reverse_combined_string("", "")==""
output: def reverse_combined_string(s1, s2):
      return (s1 + s2)[::-1]