Factor $-664888=\left(2i\right)^{2}\times 166222$. Rewrite the square root of the product $\sqrt{\left(2i\right)^{2}\times 166222}$ as the product of square roots $\sqrt{\left(2i\right)^{2}}\sqrt{166222}$. Take the square root of $\left(2i\right)^{2}$.
