Principally, (± i)²=i²=-1, so √-1=± i. But usually, the conventional definition, √-1=i, is applied to evaluate the square root of negative numbers instead of √-1=± i. So, we take √-1=i right now. (But keep in mind that you need √-1=± i in the not too distant future.)
The answer is: 9√-1=9i *
* For questions like this appeared in this site: 4√-16=4√-1√4²=16i, 3√-27=√-1√27=3√3i,
√-64=8i, but √64=±8, really confusing.