15. $$6 log_{\sqrt[6]{13}} 13$$

$$6 log_{\sqrt[6]{13}} 13 = 6 log_{13^{\frac{1}{6}}} 13 = 6 \cdot \frac{1}{\frac{1}{6}} log_{13} 13 = 6 \cdot 6 \cdot 1 = 36$$

Ответ: $$36$$