The function is .

Horizontal line test:

A function is one-to-one if and only if no horizontal line intersects its graph more than once.

Graph :

Graph the function .

Draw a horizontal line.

Observe the graph :

Draw the horizontal line .

The horizontal line touches the graph twice .

The function is a not one-to-one, because it violates the horizontal line test.

The function is a not one-to-one.