Final answer:
After four years, with a yearly depreciation expense of $3,000, the beginning net book value of the equipment is $12,000, and at the end of the fourth year, it is $9,000. The balance sheet would show the equipment at its original cost with a deduction for accumulated depreciation.
Step-by-step explanation:
To record the depreciation expense for the fourth year, you would debit the Depreciation Expense account and credit the Accumulated Depreciation account with the amount of $3,000. At the beginning of the fourth year, the net book value of the equipment is its original cost ($21,000) minus the accumulated depreciation after three years ($9,000), which equals $12,000. At the end of the fourth year, you would deduct the depreciation expense for the year ($3,000) from the beginning net book value ($12,000) to arrive at the ending net book value of $9,000.
On the balance sheet for the fourth year, the equipment will be recorded at its historical cost of $21,000, with accumulated depreciation recorded as a contra asset account to reflect the total depreciation to date ($12,000), resulting in a net book value of $9,000.