This is a basic octave.
On your fretting hand, use your index for the "5" note (D) and your ring finger or pinky for the "7" (also D, but one octave up). Lay your ring finger down a little bit so a teeny bit is touching the middle string to mute it. Then just strum or play it like a power chord.
If that explanation doesn't help, let me know and I'll do a quick video tomorrow.