input: 
  from typing import List
  
  
  def reverse_concatenate(s1: str, s2: str) -> str:
      """ Combine two strings and then reverse the combined result.
      >>> reverse_concatenate("Hello ", "world!")
      '!dlrow olleH'
      >>> reverse_concatenate("Open", "AI")
      'IA nepO'
      """
  
output:     combined = s1 + s2
      return combined[::-1]