the initial expression is:
[tex]\sqrt[]{20}-2\cdot\sqrt[]{5}[/tex]so to rest them we want to have the same radical so we can rewrite the root of 20 so:
[tex]\begin{gathered} \sqrt[]{4\cdot5}-2\cdot\sqrt[]{5} \\ 2\cdot\sqrt[]{5}-2\cdot\sqrt[]{5}=0 \end{gathered}[/tex]