Here are the two-string Rock chord shapes for A, A6 and A7. In all three chord shapes only the 5th and 4th strings are strummed. The open 5th string is the root note (A).
Play the following chord progressions using the above rock chord shapes. Use eighth note strums and only play the 5th and 4th strings. When playing power chords, it is common to use only downstrums, as this achieves the best sound.
Here are the previous examples as a two bar pattern.