Which figure correctly demonstrates using a straight line to determine that the graphed equation is not a function of x?

Lower left.

Step-by-step explanation:

To determine if a graph is a function the "vertical line test is used.  If any point on a graph has one x value go to two y values it is not a function.  

Witht he four pictures, the two on the right use a horizontal line, so it doesn't tell at all, on the leftthe upper one is vertical but it doesn't show that one x value has two corresponding y values.  So it must be the lower left one.

The lower left picture has a line at x=1, and it passes through the line of the graph at two different y values, 4 and -4.  So this has two y values with one x value.

c. (3,2,6) is the answer.