All positive numbers have a square root, and a square root is defined as the number you can multiply by itself to get the original number.
In this case 9 x 9 = 81, which means that 9 is a square root of 81. However, if you multiply two negative numbers, you get a positive number. This means that every positive number has a square root that is positive and negative.
-9 x -9 = 81
The answer is -9 and 9