Answer
$2i\sqrt{7}$
Work Step by Step
Factor the radicand so that one factor $-1$ to obtain:
$=\sqrt{28(-1)}
\\=\sqrt{4(7)(-1)}
\\=\sqrt{2^2(7)(-1)}$
RECALL:
(1) $\sqrt{abc} = \sqrt{a} \cdot \sqrt{b} \cdot \sqrt{c}$
(2) $\sqrt{-1} = i$
Use rule (1) above to obtain:
$=\sqrt{2^2} \cdot \sqrt{7} \cdot \sqrt{-1}
\\=2 \cdot \sqrt{7} \cdot \sqrt{-1}
\\=2\sqrt{7} \cdot \sqrt{-1}$
Use rule (2) above to obtain:
$=2\sqrt{7} \cdot i
\\=2i\sqrt{7}$