This exercise is from the Core Skills section on Palm Muting.

We practiced playing powerchords in the strumming exercises. Now we shall begin to apply basic palm muting to them. All rock fans must know how to play the next set of exercises.

Here we play an E powerchord. This is the simplest powerchord to play in standard tuning. In the ActionTab we use just 1 finger to hold both notes on the A and D string. This is a common way to play this powerchord because the 1st finger is naturally almost in a 'barre' shape, which means if you need to change to playing another powerchord (using the E or A shape), your fingers are already halfway to holding the first part of the next chord shape.

To use one finger to fret across 2 strings your finger needs to approach the neck from a shallow angle. Use your second finger on the D string to fret the note instead if doing this is too difficult.

Your downstrokes should be hitting all 3 strings whilst palm muting in this exercise. You need to keep your palm edge against the bottom 3 strings to make sure each string is dampened whilst you strike the powerchord. Remember you don't need to squash the strings - just rest your palm against them. Too much pressure and you will not have enough flexibility in your wrist to strum properly.