Determine if the relation defines y as a function of x.

Explanation:
This graph passes the vertical line test, so that's why we have a function.
It is impossible to draw a single vertical line to have it pass through more than one point on the blue curve. Keep in mind that open holes are not part of the graph. So a vertical line at say x = -2 only passes through (-2,0) and not (-2,-2).
In other words, the input x = -2 leads to the output y = 0 (and not y = -2).