For the net cash flow series, find the external rate of return (EROR) using the MIRR method with an investment rate of 21% per year and a borrowing rate of 8% per year. Year 1 2 3 4 Net Cash Flow, $ 3,000 -9,000 -5,000 8,000 5 -2,500 6 4,700 The external rate of return is %