This is a multi part problem, the assigned problem is 7.21, but I'm totally lost [The answers are given but i need an explanation as to how to get them]

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[7.21] Compute the capital recovery for the machine of Problem, 7.15, using (7.6). Compare with the answer to Problem 7.20 (answer to 7.20 is $22 941.26 per year). Answer $2941.25 = $22 941.26 - $20 000

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[7.15] A used machine costs $20 000 to purchase. It has an annual maintenance cost of $20 000, a salvage value of $5000, and a 10-year life. If the interest rate is 10% per year, compounded annually, what is the present-worth cost of the machine? Answer $140 960.12

[7.6] is below

Let i= 15% per year, compounded annually. Determine the present worth of the following cash flows: End of Year 0 Cash Flow, $100010 2 26 6 Analyze the last two flows as S2000+ S4000. Then, PW =-$ 10 000 + $2000(PA, 15%, 4) + $4000(PlA, 15% , 2)(HF, 15% , 2) =-$ 10 000 + $2000(0.35027)-14 $4000(0.61512)-(1.3225)--S626, 93