The answer is 18.

**
This section requires Javascript.
**

You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.

Let $N=k^{4}$ for some positive integer $k$ . Then, we get $7b^{2}+7b+7=k^{4}$ $7\mid k^{4} \Rightarrow 7\mid k$ Substituting in $k=7$ , we get the answer $b=18$ . (You can check the discriminant as well, but I left it out.)