why is root 3i multiplied by - root 3i equal to 3?

$$i=\sqrt{-1}$$        It is an imaginary number.

$$i\times i = \sqrt{-1}\times \sqrt{-1} = -1$$

SO

√3i x (-√3i) 

$$\\=\sqrt3\times i\times -\sqrt3\times i\\ =-\sqrt3\times\sqrt3\times i\times i\\ =-\sqrt9\times i\times i\\ =-3\times -1\\ =3$$