в) -3 √49a²/64b⁴ при а = 1/7, b = 3/4;

в) $$-3 \cdot \sqrt{\frac{49a^2}{64b^4}} = -3 \cdot \frac{\sqrt{49} \cdot \sqrt{a^2}}{\sqrt{64} \cdot \sqrt{b^4}} = -3 \cdot \frac{7 \cdot |a|}{8 \cdot b^2}$$.

При $$a = \frac{1}{7}, b = \frac{3}{4}$$, получим:

$$-3 \cdot \frac{7 \cdot |\frac{1}{7}|}{8 \cdot (\frac{3}{4})^2} = -3 \cdot \frac{7 \cdot \frac{1}{7}}{8 \cdot \frac{9}{16}} = -3 \cdot \frac{1}{8 \cdot \frac{9}{16}} = -3 \cdot \frac{1}{\frac{9}{2}} = -3 \cdot \frac{2}{9} = -\frac{2}{3}$$.

Ответ: $$\frac{-2}{3}$$