[tex]^2\sqrt[]{3x}+\sqrt[]{3x}\text{ =2}\sqrt[]{3x}[/tex]
Since when a radical doesn't have an index, is a square root, the first one can be simplified.
[tex]3\sqrt[]{5x}-6\sqrt[]{5x\text{ }}=-3\sqrt[]{5x}[/tex]
The second option can be simplified.
[tex]^3\sqrt[]{5}+^{}2^3\sqrt[]{5}=3^3\sqrt[]{5}[/tex]
The third one can be simplified.
[tex]5^3\sqrt[]{x}+\sqrt[]{x}[/tex]
The last option cannot be simplified further.
