The following are the examples of not propositions:
x = 5 (this is not an assertion of truth, it's an assignment)
x + y = 5 (not enough information to be an assertion, answer depends on missing data)
Propositions can combine terms using connectives (and, or not).
Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month, and much more.