Please explain me how to do this task. Thank you so much.

Question 4 - Depreciation (22 points) Northeast Airlines purchased an aircraft on January 1, 2018, at a cost of $45,000. The estimated useful life of the aircraft is 20 years, with an estimated residual value of $5,000. On January 1, 2021 the airline revises the total estimated useful life to 13 years with a revised residual value of $4,000. Required: Answer the following TWO parts. Show your calculations. (a) Compute the book value at December 31, 2020 using the double-declining-balance method. Your Answer (Required) $ Computations (Optional) Book value at Dec. 31, 2020 (b) Assuming the straight-line method is used, compute the depreciation expense for the year ended December 31, 2021. Your Answer (Required) $ Computations (Optional) Depreciation expense for the year ended Dec. 31, 2021