Added hints for 3x3 matrix inverses

Added hints for 3x3 matrix inverses; originally there were only links to the videos.
![Screen shot 2013-02-02 at 5 50 50 PM](https://f.cloud.github.com/assets/1689183/121343/05173482-6d8b-11e2-8da2-451404dac315.png)