12.4 a¹⁄₁₂⋅⁴√a.

$$ a^{\frac{1}{12}} \cdot \sqrt[4]{a} = a^{\frac{1}{12}} \cdot a^{\frac{1}{4}} = a^{\frac{1}{12} + \frac{1}{4}} = a^{\frac{1}{12} + \frac{3}{12}} = a^{\frac{4}{12}} = a^{\frac{1}{3}} $$

Ответ: a^(1/3)