Answer
$2\sqrt{x-1}$
Work Step by Step
Extracting the factor that is a perfect power of the index results to
\begin{array}{l}\require{cancel}
\sqrt{25x-25}-\sqrt{9x-9}
\\\\=
\sqrt{25(x-1)}-\sqrt{9(x-1)}
\\\\=
\sqrt{(5)^2\cdot (x-1)}-\sqrt{(3)^2\cdot(x-1)}
\\\\=
5\sqrt{x-1}-3\sqrt{x-1}
\\\\=
2\sqrt{x-1}
.\end{array}
