Sales Forecast

I would suggest that this is more of a math question than an Excel question. Yes you can predict sales using any number of functions. The hardest part is trying to decide what kind of function to use.

1) For a purely empirical approach, you can use a polynomial (via the LINEST() function or the chart trendline feature).
2) Perhaps there is seasonality or periodicity that needs to be taken into consideration. There are various ways to do this, perhaps some as simple as using a trigonemetric sin() or cos() function. These are usually non-linear regressions, but they can still be done using the Solver utility.
3) Perhaps you prefer an exponenential or logarithmic function. These, too, can be regressed using the LINEST() function.

It really depends on what kind of trend you expect to see. I'm not sure that we on this side of the internet are really in a position to make this kind of judgement. Once you decide what kind of function you want to try, I'm sure we can help you program that regression into Excel.

If you don't have any metric, maybe look into the Forecast function.

http://www.excelfunctions.net/Excel-...-Function.html

Not really. About the only thing that really changes is that I don't think and exponential type trendline is likely, but the other choices are still equally valid. Without more information, I'm not sure there is much more than can be said.

I might be tempted to do something with a straight line regression (y=m*t+b), with the explanation that this forecast looks like it would overpredict future sales and leave it at that.
If I only need a short term forecast (depending on how I want to define short term), I might be tempted to use a 2nd order polynomial. Knowing that this will predict a maximum in sales and then decreasing sales from there on, I do not know how far out I would like to use this kind of forecast.

Edit: A note on JieJenn's FORECAST() solution: the FORECAST() function uses a simple straight line regression (y=mx+b). You will still need to decide whether that is the kind of equation you want to use for the forecast.