Interpolate - HELP

Hello,
I am working on a problem and I am stuck on how to use the Interpolate function.

I have attached the excel spreadsheet that I have started.
1. I have a company and the initial cost of set up is $3,800,000.
2. It is estimated that it will generate $170,000 in cash during the first year of operation, with the yearly cash flow increasing to $980,000 by the end of year 5.
Interpolate the year 1 through year 5 cash flow values, assuming that the increase in cash follows a growth trend.
3. Then from year 5 to 10 the estimate of yearly cash flow will increase from $980,000 to $1,100,000. Interpolate the cash flow figures, assuming a liner trend in the increase in cash. From year 10 to 15 the estimated to decrease from $1,100,000 to $850,000. Interpolate the yearly cash flow figures once again assuming a linear trend in the decline.

Thank you in adance for any help!
Mary