\[7b - \frac{21b^{2}}{3b + 4} =\]
\[= \frac{7b(3b + 4) - 21b^{2} \cdot b}{3b + 4} =\]
\[= \frac{21b^{2} + 28b - 21b^{2}}{3b + 4} = \frac{28b}{3b + 4}.\]