The group of points (1, 2). (5,2), (5,4), (7,6). (11, 6) (11,8) is not a function as it doesn't satisfy the condition that there is one and only one value in the image space for every element in the domain.
We have the points (5, 2) and (5, 4), which shows two values (y=2 and y=4) for the same point in the domain (x=5).
It also happens for (11, 6) and (11,8).