Making the assumption that your problem looks like this,
[tex]\sqrt{5x}(\sqrt{x}-5\sqrt{5})[/tex]
we use the distributive property to multiply:
[tex]\sqrt{5x}\times \sqrt{x}-\sqrt{5x}\times5\sqrt{5}
\\
\\ \sqrt{5x^2} - 5\sqrt{25x}
\\
\\ x\sqrt{5}-25\sqrt{x}[/tex]
