Answer:
$67,860
Step-by-step explanation:
Depreciation = Cost - Residual amount ÷ Useful life
 = ($84,000 - $3,300) ÷ 5
 = $16,140
Book Value = Cost - Accumulated depreciation
therefore,
Book Value = $84,000 - $16,140
 = $67,860
thus
The book value at December 31, 2021, would be: $67,860