1. The problem states that the 3rd graph is not a function because it passes through 2 points with the same x-coordinate. 2. A function is defined as a relation where each input (x-value) has exactly one output (y-value). 3. If a graph passes through two points with the same x-coordinate but different y-coordinates, it violates the definition of a function. 4. This is often checked using the vertical line test: if a vertical line intersects the graph at more than one point, the graph is not a function. 5. Therefore, the 3rd graph is not a function because it fails the vertical line test by passing through two points with the same x but different y values.