remember that [tex]i=\sqrt{-1}[/tex]
[tex]3\sqrt{-24}=[/tex]
[tex]3\sqrt{(-1)(24)}=[/tex]
[tex](3)(\sqrt{-1})(\sqrt{24})=[/tex]
[tex](3)(i)(\sqrt{2*2*2*3})=[/tex]
[tex](3)(i)(\sqrt{2)(\sqrt{2)(\sqrt{2)(\sqrt{3})=[/tex]
[tex](3)(i)(2)(\sqrt{2*3})=[/tex]
[tex](6i)(\sqrt{6}=[/tex]
[tex]6i\sqrt{6}[/tex]