An adjusting entry by definition is an accounting journal at the end of an accounting period which adjust income and expenses so that they comply with the accrual basis of accounting.
In this case, since $18,000 is still unearned, therefore the amount of adjusting entry is:
Adjusting Entry = Balance – Unearned
Adjusting Entry =$72,000 – $18,000
Adjusting Entry = $54,000