Two more sets of chords like you did in the previous lesson Playing Alternating Bass Notes with D and G chords.
First practice with the chords A and D.
For the A chord use the bass notes A (the open fifth string) which is the root of the chord. For the fifth of the chord use the E (open six string).
Then practice in the same way using the G chord and the A chord.
Once you’ve done this you will have practice all the movements of the chords in this exercise. You’ll be well-prepared for exercise number 4.