as average boreduser said (kinda), your working is hella unnecessary
u can rewrite it as like
\begin{align*}
\int \frac{1}{(1-5x)^3} \text dx &=\int (1-5x)^{-3} \text dx \\
&= \(1-5x)^{-2} \cdot \frac{1}{-2} \cdot \frac{1}{-5} + C \\
&= \frac{1}{10(1-5x)^2} + C
\end{align*}