Do you mean

$$125\sqrt[3]{a^6}-\sqrt[3]{125a^6}\\\\
=125\sqrt[3]{(a^2)^3}-\sqrt[3]{5^3(a^2)^3}\\\\
=125a^2-5*a^2\\\\
=120a^2$$

125*sqrt3(a^6)-sqrt3(125*a^6)

125*a^2-5*a^2=120*a^2