Answer:
December:
Dr cash $16,000
Cr subscription revenue $16,000
Dr cash $216,000
Cr unearned revenue $216,000
January 2017:
Dr unearned revenue $18,000
Cr subscription revenue $18,000
Explanation:
The sales of 4000 copies at the newstand means that revenue of $16,000($4*4000) has been earned in December,which means that cash would debited with $16,000 and sales revenue credited with the same amount.
In December,the subscriptions received in advance for 2017 of $216,000($36*6000) would be debited to cash and credited to unearned revenue account.
At the end of January,the unearned revenue would be debited with $18,000($216,000/12) in respect of subscription earned as a result of the Magazine been mailed to subscribers in the month,which now entitles the company to one month subscription as earned sales revenue.