Answer
$2i\sqrt 7$
Work Step by Step
The number $i$ is defined as $\sqrt {-1}$
We use the rules of multiplication for square roots to separate the radical: $\sqrt {-1}\sqrt {28}$
We subsititute in $i$ for $\sqrt {-1}$: $i\sqrt {28}$
We simplify the real square root using multiplication rules: $i\sqrt {4}\sqrt 7=2i\sqrt 7$
