Forecast or Trend

I need to use a Formula/Function for forecasting next month's balance in bank, based on history and trend. To make it simple, if: Jan had a balance of $100, Feb of $200, March of $400, April of $800, I need a formula that will suggest that May will show a balance of $1,600 since we doubled our balance each month. Just to use the previious balance and multiply by 2 is not the solution. I use the doubling of the balances just for the example and to test if the formula verifies, as it is obvious what the result should be in this simplified test.

So in column A I have the month names, in column B are the bank balances.

For forecasting reasons, I need to have a good guess of what next month's balance will be, based on history and trend.

Can somebody provide such formula or correct use of the appropiate function based on this particular example? I spent hours and days with the available functions in Excel and none worked, so please do not direct me with the help pages on Trend, Forecast, Linest, etc. unless you can make them work on the scenario described above, with those exact examples. Any help will be greatly appreciated. PabloVitaver@gmail.com