Volume of a cube = length · width · height, or:
Volume of a cube = length³
We know that a cube is made up of 6 squares, so if the area of one of the squares is 9, we can take the square root to find the sides:
√9 = ±3
(and in this case, you can't have negative measurements, so it's positive.)
Now, all we have to do is cube the side length, so:
3³ = 27.
If you have any questions, feel free to ask! :)