I am faced with a project that I am truly stumped on. My objective is to take past historical parts data and use it to to forecast the demand monthly and yearly. This is just a sample of the data for parts sold per month with yearly total:

Year jan feb march april may june july....Total
2005 11 12 10 12 13 16 15.... 200
2006 10 12 9 12 12 15 10.... 210
2007 11 13 14 18 16 14 11.... 180

But the twist is that if we change the amount of parts we produce for a company per year. How do I make a program that will calculate forecast of demand if we change the total parts to produce next year to 220, based on the historical data i have stated. So inputs would be quantity of parts and another input would be how many months to calculate.

If you can help me on this i would truly appreciate it. I have to have this project done in 3 days and its a project for work lol. If you have any questions feel free to task at anytime.