input: Write a function to combine 2 strings and then reverse the result. The function should take two string inputs and return the reversed combined string.
  
  assert reverse_combine_strings("Hello ", "world!")=="!dlrow olleH"
  assert reverse_combine_strings("", "Python")=="nohtyP"
  assert reverse_combine_strings("Test", "")=="tseT"
  assert reverse_combine_strings("", "")==""
output: def reverse_combine_strings(s1, s2):
      return (s1 + s2)[::-1]