Answer
$-\sqrt 21$
Work Step by Step
Step 1: $\sqrt -3\times\sqrt -7$
Step 2: As $\sqrt -1=i$, the expression becomes $i\sqrt 3 \times i\sqrt 7$
Step 3: Since i^{2}=-1, the expression becomes $-\sqrt 3\sqrt 7$
Step 4: Simplifying, the expression becomes $-\sqrt 21$
