0xCMD, I'm not entirely familiar with all of your notation, so I apologize in advance if I overlook something important in your info.

A project's IRR (aka "rate of return", your question's objective) is that rate which creates a zero NPV, when all the cash flows are discounted by such rate.

Unless told otherwise, questions of this type usually have you assume the CFs all occur at the end of each year. Bad news: Solving for the single rate r which creates the requisite -0- NPV comes down to trial-and-error, if there are several cash flows (as in this case). Good news: Programs such as Excel can blaze through the T-and-E iterations in a blink.

Using Excel as the example (but use your favorite weapon of choice), lay out the cash flows and let the IRR function (or its Goal Seek tool, e.g.) do the high-speed heavy lifting for you.

It appears that what you've done to this point is to find the project's NPV (or "NW"), using a discount rate of 12%, a worthy objective in its own right. But what the question is asking requires you to fiddle with that discount rate until you land on a NPV of zero. Hint: the project you've described has an IRR significantly north of that 12%.

Best of luck!

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