a) $$(a^{-4})^{-3} \cdot a^{-30} = a^{-4 \cdot (-3)} \cdot a^{-30} = a^{12} \cdot a^{-30} = a^{12-30} = a^{-18} = \frac{1}{a^{18}}$$;
б) $$6a^{2}b^{-4} \cdot \frac{1}{3}a^{-3}b^{3} = 6 \cdot \frac{1}{3} \cdot a^{2+(-3)} \cdot b^{-4+3} = 2a^{-1}b^{-1} = \frac{2}{ab}$$