input: Write a python function to find the index of a substring within a string.
  assert find_substring("hello world", "world") == "Substring found at index 6"
  assert find_substring("python programming", "thon") == "Substring found at index 2"
  assert find_substring("data science", "math") == "Substring not found"
output: def find_substring(main_string, sub_string):
      index = main_string.find(sub_string)
      if index != -1:
          return f"Substring found at index {index}"
      else:
          return "Substring not found"