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.

That is, any horizontal line in the graph must cross the curve at one point only. 

Graph:

Graph the function .

Observe the graph :

Draw the horizontal line , this horizontal line touches the graph twice.

.

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

The function is not an one-to-one.