Whenever I encounter a double negative that gives me trouble, I rephrase it in my own words. PT46.S2.Q10 is a good example of this. You can rephrase "X is not indispensable" to "X is not essential."
Hey! Just browsing LR forum posts and saw no one responded to this. I wrote an explanation here: https://7sage.com/discussion/#/discussion/26042/pt-1-s3-q18-please-help