Simplify square root of -324

$\sqrt{- 324}$

Rewrite $- 324$ as $- 1 (324)$ .

$\sqrt{- 1 (324)}$

Rewrite $\sqrt{- 1 (324)}$ as $\sqrt{- 1} \cdot \sqrt{324}$ .

$\sqrt{- 1} \cdot \sqrt{324}$

Rewrite $\sqrt{- 1}$ as $i$ .

$i \cdot \sqrt{324}$

Rewrite $324$ as $18^{2}$ .

$i \cdot \sqrt{18^{2}}$

Pull terms out from under the radical, assuming positive real numbers.

$i \cdot 18$

Move $18$ to the left of $i$ .

$18 i$