You need to make use of the distributive property, along with ...
[tex]\sqrt{a}\cdot\sqrt{b}=\sqrt{ab}\\\sqrt{a}\cdot\sqrt{a}=a\qquad\text{meaning of square root}[/tex]
Then
[tex]\sqrt{7x}\left(\sqrt{x}-7\sqrt{7}\right)=\sqrt{7x}\cdot\sqrt{x}-7\sqrt{7x}\cdot\sqrt{7}\\\\=\sqrt{7}\sqrt{x}\sqrt{x}-7\sqrt{7^2x}=x\sqrt{7}-7\cdot 7\sqrt{x}\\\\=x\sqrt{7}-49\sqrt{x}[/tex]
