Question

How do you know if it is a function ​

Answer 1

A WAY easier (and faster), way to know if it is a function is to see if there are two of the same x-intercept (which make a vertical line). If there is, then it is NOT a function.

Answer 2

use the straight line test

Explanation:

use your pencil or draw a line horizontally (like this -> I )anywhere on the graph. If it's a function, it will only touch the line at one point no matter where you put the line.