Answer:
who will break up with whom
Step-by-step explanation:
In Brad Leithauser's poem "A Teenage Couple," the difficult topic that the couple is discussing is who will break up with whom.
The couple, according to the poem are at odds with each other and as a result we're discussing their breakup, even though they still loved each other, and the the difficult topic they were discussing is who will break up with whom.