input: Write a python function to reverse the concatenation of two strings.
  assert reverse_concatenation("Hello ", "world!") == "!dlrow olleH"
  assert reverse_concatenation("Good ", "morning") == "gninrom dooG"
  assert reverse_concatenation("Python", " Programming") == "gnimmargorP nohtyP"
output: def reverse_concatenation(str1, str2):
      concatenated_str = str1 + str2
      return concatenated_str[::-1]