Please tell me how to do it not just answer

88. Equipment was purchased on January 1 for $39,000 with an estimated residual value of $3,000. The current year's Depreciation Expense is $4,000, calculated on the straight-line basis, and the balance of the Accumulated Depreciation account at the end of the year is $12,000. The remaining useful life of the equipment is

(a) 3 years. (b) 5 years. (c) 6 years. (d) 9 years.