The imaginary number 'i" can be written in another form below:
[tex]i=\sqrt[]{-1}[/tex]Whenever we have the root of a negative number, we use the imaginary number 'i' to write it in an easier form.
For example:
[tex]\begin{gathered} \sqrt[]{-4}=\sqrt[]{-1\times4} \\ =\sqrt[]{-1}\times\sqrt[]{4} \\ =2\times\sqrt[]{-1} \\ =2\times i \\ =2i \end{gathered}[/tex]Therefore:
[tex]\begin{gathered} 84i=84\sqrt[]{-1} \\ =\sqrt[]{84^2\times-1} \\ =\sqrt[]{-7056} \end{gathered}[/tex]