Question

How to tell if a graph is a function?

Answer 1

If you take a vertical line and place it on the graph, then no matter where on the graph you place the vertical line, the graph should only cross the vertical line once. If so, then it is a vertical line.

If not, then it isn't one.

Explanation:

Answer 2

Look at one of the graphs you have a question about. Then take a vertical line and place it on the graph. If the graph is a function, then no matter where on the graph you place the vertical line, the graph should only cross the vertical line once.