Answer
$i\sqrt{21}$
Work Step by Step
Factor the radicand so that one factor $-1$ to obtain:
$=\sqrt{21(-1)}$
RECALL:
(1) $\sqrt{ab} = \sqrt{a} \cdot \sqrt{b}$
(2) $\sqrt{-1} = i$
Use rule (1) above to obtain:
$=\sqrt{21} \cdot \sqrt{-1}$
Use rule (2) above to obtain:
$=\sqrt{21} \cdot i
\\=i\sqrt{21}$
