no excel please. Equations only

3. A company is considering the purchase of a new piece of testing equipment which is expected to produce $8,000 additional before-tax profit during the first year of operation; this amount will probably decrease by $500 per year for each additional year of ownership. The equipment costs $20,000 and will have estimated salvage value of $3,000 after 8 years of use. For a before-tax interest rate of 25%, determine the net present worth of this investment