I'm very confused on how these last two methods work. Please show step by step solutions on how to find the depreciation expense for them. Thank you!

Sheffield Corporation purchased a new machine for its assembly process on August 1, 2020. The cost of this machine was $158,100. The company estimated that the machine would have a salvage value of $14,100 at the end of its service life. Its life is estimated at 5 years, and its working hours are estimated at 22,500 hours. Year-end is December 31. Compute the depreciation expense under the following methods. Each of the following should be considered unrelated. (Round depreciation rate per hour to 2 decimal places, e.g. 5.35 for computational purposes. Round your answers to 0 decimal places, e.g. 45,892.) (a) Straight-line depreciation for 2020 $ 12000 (b) Activity method for 2020, assuming that machine usage was 800 hours $ 5120 (c) Sum-of-the-years'-digits for 2021 $ (d) Double-declining-balance for 2021 $