\[\frac{4b}{3b - 21} + \frac{3b}{14 - 2b} =\]
\[= \frac{4b^{\backslash 2}}{3 \cdot (b - 7)} - \frac{3b^{\backslash 3}}{2 \cdot (b - 7)} =\]
\[= \frac{8b - 9b}{6 \cdot (b - 7)} =\]
\[= - \frac{b}{6 \cdot (b - 7)} = \frac{b}{42 - 6b}\]