First, divide all sides by 3. This will give -1/3 < r < 1/3. Here is a place where you can learn more about interval notation: https://www.varsitytutors.com/hotmath/hotmath_help/topics/interval-notation
Can you write -1/3 < r < 1/3 in interval notation?
PS Is anybody else getting this problem where they have to reload the site every single time they answer a question? Also, is it just me, or do people have to go into someone's profile to like their post, since they can't do it on the forum?
:)