Please explain steps

15. Equipment with a 4-year useful life was purchased on January 1, 2005, for $60,000. The equipment was depreciated using the straight-line method with an estimated salvage value of $12,000. On January 1 2007, the equipment was upgraded by an expenditure of $10,000 and the salvage value was increased to $14,000 (the 4-year life was unchanged). The depreciation expense for 2007 was: a $15,000 b. $12,000 e. All answers a-d are wrong c. $23,000 d. $ 16,000