\[\boxed{\mathbf{785}\mathbf{.}}\]
\[1)\log_{3}x = 4\log_{3}a + 7\log_{3}b\]
\[\log_{3}x = \log_{3}a^{4} \cdot b^{7}\]
\[x = a^{4}b^{7}.\]
\[2)\log_{5}x = 2\log_{5}a - 3\log_{5}b\]
\[\log_{5}x = \log_{5}\frac{a^{2}}{b^{3}}\]
\[x = \frac{a^{2}}{b^{3}}.\]
\[3)\log_{\frac{1}{2}}x = \frac{2}{3}\log_{\frac{1}{2}}a - \frac{1}{5}\log_{\frac{1}{2}}b\]
\[\log_{\frac{1}{2}}x = \frac{a^{\frac{2}{3}}}{b^{\frac{1}{5}}}\]
\[x = \frac{\sqrt[3]{a^{2}}}{\sqrt[5]{b}}.\]
\[4)\log_{\frac{2}{3}}x = \frac{1}{4}\log_{\frac{2}{3}}a + \frac{4}{7}\log_{\frac{2}{3}}b\]
\[\log_{\frac{2}{3}}x = \log_{\frac{2}{3}}\ a^{\frac{1}{4}} \cdot b^{\frac{4}{7}}\]
\[x = \sqrt[4]{a} \cdot \sqrt[7]{b^{4}}.\]