To determine if the relation given in the graph is a function or not, you consider the following definition:
A function is a relation one-to-one. This means that a specific value of the domain (for example a specific value of x) can not have associated two values of the range (two values of y).
You can notice in the grap tha for one value of the domain, specifically, for x = 10, you have two values of y. This situation breaks down the realtion one-to-one of a function.
Hence, the given relation is not a function.
