What is the concept of a “substitute” in economics?

You are evaluating two different silicon wafer milling machines. the techron i costs $285,000, has a three-year life, and has pretax operating costs of $78,000 per year. the techron ii costs $495,000, has a five-year life, and has pretax operating costs of $45,000 per year. for both milling machines, use straight-line depreciation to zero over the project’s life and assume a salvage value of $55,000. if your tax rate is 24 percent and your discount rate is 11 percent, compute the eac for both machines.