We have
[tex]\sqrt[]{3}\times\sqrt[]{21}[/tex]we can reduce the expression
[tex]\begin{gathered} \sqrt[]{3}\times\sqrt[]{21} \\ \sqrt[]{3}\times\sqrt[]{3\times7} \\ \sqrt[]{3}\times\sqrt[]{3}\times\sqrt[]{7} \\ 3\sqrt[]{7} \end{gathered}[/tex]the solution is
[tex]3\sqrt[]{7}[/tex]