This particular riff is played for a large portion of the song. What I really like about it is how it's played identically over three sections, yet each section sounds unique. This is in large part due to the other instruments, especially the variated parts that the drums are playing.
When you look at the tab, you'll notice that there are quite a few palm-muted 3-note groupings on the 6th string. Keep those palm mutes tight as you transition from the aggressive palm-muted notes to the accented notes on the A and D strings.
Once you have it down, I'll see you in Exercise 3!