Not using excel. What formulas are needed to solve?

A gas turbine purchased two years ago, has its market value as of now to be $260,000 and may be used for another 3 years. Its salvage value which decreases by 15% per year, is described by the equation S = 260000 (1 - 0.15)^n. The annual operating cost of the turbine will be constant at $90000. A challenger will cost $130,000 with an economic life of 6 years and an operating cost of $100,000 per year. Its salvage value will be $45,000. On the basis of these estimates and at an interest rate of 12% per year, what is the: Economic service life (ESL) of the defender? what market value for the existing asset will render the challenger equally attractive A gas turbine purchased two years ago, has its market value as of now to be $260,000 and may be used for another 3 years. Its salvage value which decreases by 15% per year, is described by the equation S = 260000 (1 - 0.15)^n. The annual operating cost of the turbine will be constant at $90000. A challenger will cost $130,000 with an economic life of 6 years and an operating cost of $100,000 per year. Its salvage value will be $45,000. On the basis of these estimates and at an interest rate of 12% per year, what is the: Economic service life (ESL) of the defender? what market value for the existing asset will render the challenger equally attractive