Which of the following is not a function? a. (1,1),(2,2), (3,3), (4,4)
b. (2,3), (2,4), (3,3), (3,4)
c (2,4),(4,8).(10,12). (4,10)
d. (2,4), (3,4), (5,4), (6,4)

B

Number on the left: x-value
Number on the right: y-value

If there are 2 y-values for one x-value it is NOT considered a function.

For instance in the

(2,3), (2,4), (3,3), (3,4)

You can clearly see that the x-values (2 & 3) repeat more than once; therefore, it is NOT a function.

On the other hand if you have 2 DIFFERENT x-values for the same y-value it IS A FUNCTION.

For ex D: (2,4), (3,4), (5,4), (6,4)

As you can see there are 4 DIFFERENT x-values with same y-value; therefore it is A FUNCTION

To make it easier:

If x-values appear more than once = NOT A FUNCTION

If x-values appear only ONCE = FUNCTION

Remember: y-values are ALLOWED to repeat

I hope this helps. If you have any questions please let me know in the comments.