If two different x's give you the same y, then it is NOT a function of x. For example, the middle photo on the left side of your question shows -3 and 6 pointing to -2. This means it's not a function. As another example, both the bottom left and top right answers are not functions because every x value matches with 2 different y values. You can tell by using the "vertical line" test. If you can draw a vertical line at any point on the graph and it hits 2 or more points on the given line, it's NOT a function of x.
TLDR is that the middle right and the bottom right are both functions of x. All the others are not functions of x.
Fun fact, the bottom left answer is an inverse function.