The basic requirement is that each value of should correspond to only one value for , not multiple values for . There can not exist two points (, ) and (,) on the graph (or satisfying the equation) with not equal to .
how to test it:
The value of a point where a vertical line intersects a function represents the input for that output value. If we can draw any horizontal line that intersects a graph more than once, then the graph does not represent a function because that value has more than one input.
as you can see, there is horizontal line that intersects a graph more than once
so, you have a function which is injective (one-to-one) function because each possible element of the codomain is mapped to by at most one argument
