Use the cash flow estimating equation for both options. For the "Buy" option, $-$ T$*($Dep$)$ will be $0$ since

this option requires no capital investment. For the "Make" option, you will have the same cash outflow in

years $1$ through $10($without a depreciation tax shield, $T^{**}($Dep$)$ will be $0$ because you depreciated the

equipment immediately making cash flow year $0$ equal to $-150,000**(1-21\%)-30,000.$ In year $10,$ you

recover the $$30,000$ of working capital. You can use the $"$PV$"$ Excel function to find the NPV of the $10$

years of cash flows even though the year $10$ cash flow value is $$30,000$ higher than cash flows $1$ through

$9$ due to the recovery of the working capital. The function is PV$($ ratemperpmt.$($fy$)).$ The rate is $15\%,$ you

have $10$ cash flows, and for put insert the value you obtain from the cash flow estimating equation. For

the future value argument, insert the $$30,000$ working capital recovery. Also works on your fin

calculator.