б) lgx=1lg 5a-3 lgb+4 lg c;

б) $$lg x = \frac{1}{2} lg 5a - 3 lg b + 4 lg c$$

$$lg x = lg (5a)^{\frac{1}{2}} - lg b^3 + lg c^4$$

$$lg x = lg \sqrt{5a} - lg b^3 + lg c^4$$

$$lg x = lg \frac{\sqrt{5a}}{b^3} + lg c^4$$

$$lg x = lg (\frac{\sqrt{5a}}{b^3} \cdot c^4)$$ $$x = \frac{c^4 \sqrt{5a}}{b^3}$$

Ответ: $$\frac{c^4 \sqrt{5a}}{b^3}$$