Regarding posting a URL as a hint to students, check out http://webwork.maa.org/wiki/AnswerHints for some options on providing hints to students. You should be able to add the following code to your problem:
$showHint = 2;
BEGIN_HINT
Watch \{ htmlLink( "http://www.youtube.com/videolink", "this video" ) \} for help with this problem.
END_HINT
This is untested, but should show the hint after 2 attempts.