Answer
$8i$
Work Step by Step
Step 1: $\sqrt (-64)$
Step 2: $\sqrt (-1\times64)$
Step 3: $\sqrt -1 \times \sqrt 64$
Step 4: Since $\sqrt -1=i$ and $\sqrt 64=8$, the expression becomes $i\times8$
Step 5: Simplifying, the final answer is $8i$