Message and promise communicate what your book is about and why readers should care. Read my post on how promise and message are different, consider the analysis you did on your audience (or go back to Writing Prompt #2), and and then:
1. Describe the core message of your book in fifty words or less.
2. Describe the promise of your book in fifty words or less.
I can almost guarantee that what you first write down will not be your final message or promise (in fact, Writing Prompt #5 and others will cover that eventuality). But you have to start somewhere to give yourself direction.