I'm trying to warn you that this isn't going to be an easy chapter. You're going to have grip the book firmly and keep a sharpened pencil nearby. First let's take stock of what you know. To agree with nouns, which may be in all the possible cases, numbers and genders, adjectives must be able to decline. The name speaks for itself.
Some adjectives get their case endings from the third declension.
Let's take up the first point by reviewing the third declension endings for nouns. An adjective is going to have to able to modify nouns of all three genders, so a third declension adjective will have to be able to masculine, feminine, and neuter. Cover up the two columns of endings on the right and try to write down the endings third declension adjectives are going to use. Check your work against the answers given in the two right columns. So these are the variable case endings which are going to be attached to the stem of third declension adjectives.