why is 3938 the right answer?

Question 18 0/1 pts Russin Tax Planning Service bought computer equipment for $21,000 on January 1, 2016. It has an estimated useful life of four years and zero residual value. Russin uses the straight-line method to calculate depreciation and records depreciation expense in the books at the end of every month. Calculate the amount of Depreciation Expense for the period, January 1, 2016 through September 30, 2016, for this equipment. (Round any intermediate calculations to two decimal places, and your final answer to the nearest dollar.) KNOW WHAT ACCOUNT GETS CREDITED AS WELL. You Answered $5250 Correct Answer $3938 $5688 $4375