You don't have enough information to do more than estimate the number of new visits that bounce -- and I think you've done it correctly.
1) First you calculate the number of new visits, which you correctly identified as 2683.9 (3909 * 68.66%).
2) Then -- and this is where the lack of information comes into play -- you have to assume that the bounce rate of new visits and the bounce rate of total visits is the same. In fact, it may not be, but since the total visit bounce rate is all we have, then all we can do is assume that it is. Therefore, just multiply the number of new visitors in step 1 by the total bounce rate to get the number of new visitors that bounced. So:
2683.9 * 56.13% = 1506.5
And that's your answer.