Ok, so
Here we have the expression:
[tex]-27\sqrt[]{3}+3\sqrt[]{27}[/tex]Remember that:
[tex]\sqrt[]{27}=\sqrt[]{3\cdot3\cdot3}=\sqrt[]{3^2\cdot3}=3\sqrt[]{3}[/tex]If we rewrite:
[tex]\begin{gathered} -27(\sqrt[]{3})+3(3\sqrt[]{3)} \\ =-27\sqrt[]{3}+9\sqrt[]{3} \end{gathered}[/tex]If we have the same radical, we can sum the terms:
[tex](-27+9)\sqrt[]{3}=-18\sqrt[]{3}[/tex]