This is not an answer to the question, but rather we need some more information.
These look like coordinates on a graph - is this homework related to graphs? Maybe having to do with finding the slope, or finding the distance between two points.
It might also be a question about ratios and proportions. It might be that (3,y)(1,9) means 3 is to y as 1 is to 9, or 3:y=1:9, which could also be written as 3/y = 1/9, in which case y=27.
Please respond with some more information about the subject of this homework.