Ответ:

\(b=\frac{41}{5}\), а \(36a^2+12ab+b^2=(6a+b)^2\).

\(\sqrt{(6a+b)^2}=|6a+b|=\left|6\cdot\frac45+\frac{41}{5}\right|=\frac{65}{5}=13\).

Ответ: \(13\).