Visit Google and try a search for subtracting witte (http://google.com/search?q=subtracting+witte). You should come across a video called Subtracting Integers by the Witte Math Program (http://blip.tv/file/1837295). It carries a Creative Commons Attribution License, so we are free to use it as long as we provide an attribution link. Follow the link in the search results to Blip.tv now and preview the video.
We are going to embed this video in our Moodle course: