I have the exercise, the most thing i need is the formulas

Chapter 9 Plant Assets (a) Faster Company purchased equipment in 20x1 for $104,000 and estimated an $8,000 salvage value at the end of the equipment's 10-year useful life. On December 31, 20x7, there was $67,200 in the Accumulated Depreciation account for this equipment using the straight-line method of depreciation. On March 31, 20x8, the equipment was sold for $21,000. (b) Lewis Company sold equipment for $11,000. The equipment originally cost $25,000 in 20x1 and $6,000 was spent on a major overhaul in 20x4 (charged to the Equipment account). Accumulated Depreciation on the equipment to the date of disposal was $20,000. (c) Selby Company sold equipment that had a book value of $13,500 for $15,000. The equipment originally cost $45,000 and it is estimated that it would cost $57,000 to replace the equipment