Question

How do i figure out which table represents a function?

Answer 1

We need to find a relationship between the data points in the table. Let's say the table is
x | y
----
1 | 2
2 | 4
3 | 6
Then the function would be y = 2x

If the table were
x | y
----
1 | 1
2 | 4
3 | 9
Then the function would be y = x^2

Answer 2

If one value of x gives you more than one, or zero values of y back, then it is not a function.
If each value of x gives you one value of y, then it is a function.
Even if it gives you the same number for each x, like f(x)=0 for all x.
This is still a function so long as one value of x gives you only one value of y in return.

For instance

x | y
----
0| 1
2| 2
3| 2
5| 5

is a function, but

x | y
----
1|-1
0| 5
0| 2
4| 2

is not, because it gives two y values for the value x=0