The Week 5 lectures are available. They are about lists. It's too bad these lectures were not available earlier. They would have made this week's exercises easier.  

These are important concepts. Watch videos 5.1 - 5.6 and be sure you understand them. It may require that you watch some of the videos multiple times before you fully get them.

Videos 5.7 and 5.8 are about program correctness. These too are important concepts, but they aren't required for doing any of the exercises.