jmc

The first thing to do is place the B since it has a restriction on it. We can put it anywhere but the first place, so we have 5 options. Once we have done that, we just need to place the two N's and then the rest of the spots will just be A's. We have 5 spots left, so there are 5 options for where to put the first N and 4 options for where to put the second N. However, the two N's are identical, which means that we've counted each new word twice. So our answer is $\frac{5\times 5\times 4}{2}=\boxed{50}$.