Answer:
[tex]{ \boxed{ \mathfrak{ \: answer}}} : [/tex]
[tex]{ \tt{ \sqrt{ - 84} = \sqrt{ - 1 \times 4 \times 21} }} \\ \\ = { \tt{ \sqrt{ {(i}^{2}) } \times \sqrt{4} \times \sqrt{21} }} \\ \\ { \boxed{ \boxed{ \rm{ \: \: answer = 2i \sqrt{21} = 9.165i }}}}[/tex]
