Watch the Slide Second online guitar lesson by Brad Carlton from Chord Studies: Open String Chords Vol. 2
This is a bluesy I-IV vamp in the key of A. The first chord is an A9 with no 3, which you could view as an Em over an A bass. Moving to the D/A your third finger will act as a guide finger. Now you pick everything up except your second finger, which will guide you into the A7 chord. For this progression it's best to finger this chord with fingers one and two.