13. Вычислите: $$\frac{1}{2} log_7 36 - log_7 14 - 3 log_7 \sqrt[3]{21}$$ ;

$$\frac{1}{2} \log_7 36 - \log_7 14 - 3 \log_7 \sqrt[3]{21} = \log_7 36^{\frac{1}{2}} - \log_7 14 - \log_7 (21^{\frac{1}{3}})^3 = \log_7 \sqrt{36} - \log_7 14 - \log_7 21 = \log_7 6 - \log_7 14 - \log_7 21 = \log_7 \frac{6}{14 \cdot 21} = \log_7 \frac{6}{294} = \log_7 \frac{1}{49} = \log_7 7^{-2} = -2$$

Ответ: -2