3) log, 36 - log7 14 - 3 log721; 4) 2 log 16-log1 400 + 3 log 1 45.

3) $$\frac{1}{2} log_7 36 - log_7 14 - 3 log_7 \sqrt[3]{21} = log_7 36^{\frac{1}{2}} - log_7 14 - log_7 (\sqrt[3]{21})^3 = log_7 6 - log_7 14 - log_7 21 = log_7 \frac{6}{14 \cdot 21} = log_7 \frac{3}{7 \cdot 21} = log_7 \frac{1}{49} = log_7 7^{-2} = -2$$

4) $$2 log_{\frac{1}{2}} 6 - log_{\frac{1}{2}} 400 + 3 log_{\frac{1}{2}} \sqrt[3]{45} = log_{\frac{1}{2}} 6^2 - log_{\frac{1}{2}} 400 + log_{\frac{1}{2}} (\sqrt[3]{45})^3 = log_{\frac{1}{2}} \frac{36 \cdot 45}{400} = log_{\frac{1}{2}} \frac{9 \cdot 45}{100} = log_{\frac{1}{2}} \frac{9 \cdot 9}{20} = log_{\frac{1}{2}} \frac{81}{20}$$

Ответ: -2; $$log_{\frac{1}{2}} \frac{81}{20}$$