\[\sqrt[3]{6} \cdot \sqrt[3]{36} + \frac{\sqrt[7]{81^{4}}}{\sqrt[7]{9}} =\]
\[= \sqrt[3]{6 \cdot 36} + \sqrt[7]{\frac{81^{4}}{9}} =\]
\[= \sqrt[3]{216} + \sqrt[7]{9^{7}} = 6 + 9 = 15\]