Answer
See below.
Work Step by Step
A relation (a set of ordered pairs) can not be a function if there exist inputs with more than one output.
A graph represents a set of ordered pairs (points' coordinates.)
This means that if a vertical line intersects the graph in more than point, we have more outputs for one input of x,
eliminating the possibility of the graph representing a function.
If we prove that ALL vertical lines have AT MOST one intersection with the graph,
then it is a graph of a function.
