Answer and Explanation:
The Journal entry is shown below:-
On December 31, 2018
Unearned rent revenue Dr, $12,600 ($21,600 × 7 months ÷ 12 months)
      To Rent Revenue $12,600
(Being unearned rent is recorded)
Here for recording unearned rent, Â we debited the unearned rent revenue as it decreased the liabilities and we credited the rent revenue as it increased the revenue and the same is to be considered