If we want to check that a graph is function or not, we have a way to check by doing these steps.
Draw a vertical line, make sure that a line has to pass through or intercept a graph.
See if a line intercepts a graph more than once.
If a line intercepts a graph only one point, a graph is indeed a function. Otherwise, not a function but a relation instead. That includes if a line intercepts more than a point which doesn't make a graph a function.
From the graph, if we follow these steps, we will see that a line will only pass or intercept the graph only one point. Hence, the graph is indeed a function. The following graph that is shown is called "Parabola" for a < 0.
