From a tabular values of x and y, for it to be a function, every x coordinate has to have exactly one y coordinate. No repetitions.
Looking at our table:
We see that there are 6 different x values.
Are there any repetitive y values?
YES!
Both x = 5 and x = -5 gives the same y value of "18". Thus, this can't be a function.
Answer:
This table doesn't represent a function because y-values ARE repeated.