Can someone explain to me why we are using $17,400 and not $8,700 as the answer?

The way I would have solved this problem would have been:

Rent Revenue (Dr.) 8,700

Unearned Rent Revenue (Cr.) 8,700

Adjusting entries with deferrals: Problem type 2 On November 1, Rivera Company received $26,100 for six months of rent in advance. On November 1, Rivera Company credited Rent Revenue, which is an alternate way of recording the initial receipt of cash. Required: Journalize the adiusting entry on December 31