A poem is any work of art that exploits some of the musical characteristics of language, such as meter, rhythm, euphony, and rhyme.
Conditionals
Work of art + → poem Exploits some musical qualities of language
Limerick → ~art
Other Facts
Novels usually don't exploit musical characteristics of language. Symphonies usually don't involve language.
Evaluation
This is kind of a mess. When we read that novels don't usually exploit the musical characteristics of language (after having just learned that this is one of the definitional qualities of a poem), we could infer that novels are usually not poems. Technically, that looks like we're doing an illegal negation (if you're not the left side, you're not the right side), but in context the first sentence feels to me like a bi-conditional.
Since it's definitional, it seems fair to say that if you fit that definition, you are a poem and if you don't fit that definition you aren't a poem.
We could similarly infer that limericks are not poems, since they don't have the definitional ingredient of being a work of art.
Goal
Because of how messy the facts are, I would primarily think about the two conditionals we were given. We have the power to prove something is a poem. We have the power to prove something isn't art.