Your solution looks solid. There must be a problem with the question. Just trying values in the options show that you are correct.
7: 49 < 18-21 -- Nope.
-7: 48 < 18+21 -- Nope.
4: 16 < 18-12 -- Nope.
etc.
You can rule out all the options on the page.
Man, you've been having a string of bad luck with your textbooks containing faulty problems. If the problems are this bad, I wonder if the material is equally flaky?