TeX source:
\begin{array}{rcl}\mathbb{D}&=&\mathbb{R}\setminus_{\{0\}} \cr \cr f(x) &=& 15 \cdot x^{-\frac{6}{5}} \cr \cr \cr f'(x) &=& 15 \cdot \left(-\dfrac{6}{5}\right) \cdot x^{-\frac{6}{5}-1} \cr \cr &=& -18 \cdot x^{-\frac{11}{5}} \cr \cr &=& -18 \cdot \dfrac{1}{\sqrt[5]{x^{11}}} \cr \cr &=& -\dfrac{18}{\sqrt[5]{x^{11}}} \end{array}