Since she used 12 marshmallows and 8 graham crackers to make the four s'mores, you can figure out that she used 2 graham crackers per s'more (8/4) and 3 marshmallows (12/4).
To keep the same amount of materials per s'more you would need 2 x 3 = 6 (so 6 graham crackers) and 3 x 3 = 9 ( so 9 marshmallows).
