def FIND(find_text, within_text, start_num=1):
  """
  Returns the position at which a string is first found within text.

  Find is case-sensitive. The returned position is 1 if within_text starts with find_text.
  Start_num specifies the character at which to start the search, defaulting to 1 (the first
  character of within_text).

  If find_text is not found, or start_num is invalid, raises ValueError.

  >>> FIND("M", "Miriam McGovern")
  1
  >>> FIND("m", "Miriam McGovern")
  6
  >>> FIND("M", "Miriam McGovern", 3)
  8
  >>> FIND(" #", "Hello world # Test")
  12
  >>> FIND("gle", "Google", 1)
  4
  >>> FIND("GLE", "Google", 1)
  Traceback (most recent call last):
  ...
  ValueError: substring not found
  >>> FIND("page", "homepage")
  5
  >>> FIND("page", "homepage", 6)
  Traceback (most recent call last):
  ...
  ValueError: substring not found
  """
  return within_text.index(find_text, start_num - 1) + 1