\(⇒\) \( - \frac{d}{{dx}}\left[ {\frac{a}{{{x^{12}}}} - \frac{b}{{{x^6}}}} \right] = 0\)
\(⇒\) \( - 12a{x^{ - 13}} + 6b{x^{ - 7}} = 0\)
\(⇒\) \(\frac{{12a}}{{{x^{13}}}} = \frac{{6b}}{{{x^7}}}\)
\(⇒\) \(\frac{{2a}}{b} = {x^6}\)
\(⇒\) \(x = \sqrt[6]{{\frac{{2a}}{b}}}\)