It is not a function if one of the xs has 2 ys associated with it. The graph below shows the definition rather clearly. The red parabola opens right. There is a blue vertical line that shows the 2 y values that the red graph has.
the green line is a function. Each x has only 1 y associated with it.
Answer
The red line is not a function.
The blue line shows why not (x = 5 has 2 y values).