It is 3i.
The square root of -9 is the square root of -1 times the square root of 9.
So: the square root of 9 is 3, and the square root of -1 is called i (this doesn't actually exist, it's just imaginary).
Then, the square root of -9 is 3i.
Answer:

<em><u>I </u></em><em><u>hope </u></em><em><u>it </u></em><em><u>helps</u></em>
<em><u>have </u></em><em><u>a </u></em><em><u>great </u></em><em><u>day </u></em>
<em><u>
</u></em>