Question 42161
You do the "vertical line test."
And yes, you can use your pencil.
Lay your pencil straight up and down on the paper on the graph of the "function."
If your pencil crosses the "function" MORE THAN ONCE, it is NOT a function, but only a relation.
The idea for a function is, that for any x you choose, ONLY ONE y exists.
Good luck on the final.