a: \(6^7=2^7\cdot3^7\)
\(12^5=2^{10}\cdot3^5\)
=>\(\dfrac{6^7}{12^5}=\dfrac{1}{8}\cdot9=\dfrac{9}{8}>1\)
=>6^7>12^5
b: \(\dfrac{\left(0.4\right)^4}{\left(0.8\right)^3}=\left(\dfrac{2}{5}\right)^4:\left(\dfrac{4}{5}\right)^3=\dfrac{2^4}{5^4}:\dfrac{2^6}{5^3}=\dfrac{5}{4}>1\)
=>\(\left(0.4\right)^4>\left(0.8\right)^3\)