Use your pinky to get the 6! This last riff can be used on different string for scales which don’t have open bass strings. Just shift it to the right position on the fretboard and play!
The following riff uses the notes of a E/E6/E7 chord progression, which is also used in rock music:
E/E6/E7
How To Play 7 Nation Army On Acoustic Guitar For Beginners How To Play Acoustic Guitar For Beginners 5k