Since we are finding three arithmetic means between 7 and 21, that means there are 4 intervals between 7 and 21. The difference between 21 and 7 is 14, which we divide by 4 to get the value of each interval. Since each interval is 14/4 or 3.5, we keep adding that value to 7 to get our three arithmetic means. They are 10.5, 14, and 17.5