In Exercise 2, we'll start out by playing an F octave chord on the A and G strings. Notice the "X" symbol on the D string? That simply means you'll be muting the D string while you strum the octave chords. Make sure your strumming stays relaxed as you switch between the 8th notes and 16th notes.
Lastly, be mindful of the palm mutes throughout the riff. You'll see a "P.M." marking above any notes you're supposed to palm mute.