A function is called being one-to-one if, for every value of x, there is only one value of y and vice-versa.
If the graph of the function is given, a practical rule to determine if the function is one-to-one is to use the horizontal-line test.
This test is as follows: Imagine you have a horizontal line (maybe a ruler) and you can move it up and down the grid.
If your line touches the graph only once at every vertical position, then your function is one-to-one.
Now if we test our function drawn in blue, we can touch it only once as we move our line up and down, thus:
Is f one-to-one?
Yes
