How would I recoed an entry for dec.31st , for a policy bought for 7,200 on October 1st?

if the policy is for insurance I would Debit Prepaid Insurance for the amount and Credit Cash (if cash was paid) or Accounts Payable (if on account) on October 1.

if there were no other transactions posted during the accounting period then I would figure out how much of the policy was used.

If the Policy was for 1yr then I would take the $7,200 X 3(months Used)/12(months in a year) which equals $1,800.

The adjusting entry in this case is:
Debit the Insurance Expense account for 1,800
Credit the Prepaid Insurance account for 1,800