I invest �A in a bank at a rate of interest of 3.5% per annum. How long will it be before i double my money?

I used the formula nth term= a x r^(n-1) , and i got 21.15, but in the solution the answer was 20.15 and they used a x r^n . I don't understand when to use they used this formula rather than the one i used - could someone explain? Thanks