You cannot have a negetive number inside a cube root. So we use an "i" instead.
This is how you do it:
[tex]5 \sqrt[3]{-27} + 18[/tex] [tex] \sqrt[3]{-27} [/tex] becomes [tex]3i[/tex]
[tex]5(3i) + 18[/tex] multiply
[tex]15i + 18[/tex] reduce
[tex]3(5i + 6)[/tex]