This week you need only watch the first two videos (2.1 and 2.2). 

Video 2.3 applies the ideas in 2.1 and 2.2. It's worth watching, but there won't be questions about it on the quiz.

Video 2.4 is a summary of the Scala syntax used so far. You may find that useful, but there won't be any questions about it on the quiz.

The others are about new topics, which we'll do next week.

You should do all  the Set programming exercises. In doing the programming exercises note that the wiki points to a forum message from last year. It still applies. Basically it says that you should be careful not to confuse the Scala Set in the programming exercise with the Java Set from the Java Collections library.