Ok, so you didn't put any picture to go along with the question but hey, thats ok because I can still help you.
In order for a graph to be positive, it has to be leaning to the right.
In order for a graph to be negative, it has to be leaning to the left.
In order for a graph to be zero, it has to be horizontal.
In order for a graph to be undefined, it has to be completely vertical.
You can look at the picture below if you still need help.
P.S. The picture with the yellow lines is the ones that are zero and undefined. The yellow line on the left is undefined, the yellow line on the right is zero.
