input: Write a function to concatenate and reverse two given strings.
  assert reverseString("Hello", "World") == "dlroWolleH"
  assert reverseString("Python", "Programming") == "gnimmargorPnohtyP"
  assert reverseString("abc", "def") == "fedcba"
output: def reverseString(str1, str2):
      concatenated_str = str1 + str2
      reversed_str = concatenated_str[::-1]
      return reversed_str