\[\left( \frac{b + 1}{b - 1} - \frac{b}{b + 1} \right)\ :\frac{3b + 1}{2b - 2} =\]
\[= \frac{b^{2} + 2b + 1 - b^{2} + b}{b + 1} \cdot \frac{2}{3b + 1} =\]