we know that
A function is a relation for which each value from the set the first components of the ordered pairs is associated with exactly one value from the set of second components of the ordered pair
that means
For one value of x , there is exactly one value of y
Looking at the graph
For one value of x , there are two values of y
therefore
Is not a function
To better understand the problem i will attached a figure
please wait a minute
Example
For x=5
there are y=5.5 (approximate) and y=-5.5 (approximate)
