Robles Construction Company purchased a machine that cost $37,000 and had an estimated residual value of $1,000. The machine's useful life was 5 years, or 7,500 hours. It was used for 2,100 hours this year. Compute depreciation expense under the following methods: Straight-line Depreciation Expense = Units-of-production Depreciation Expense = Double Declining Balance Depreciation Expense =