For a function, you want each x-value to have one unique output, 1 passes that because we don't see any repeating x-values. 2 isn't a function because the point (9,5) and (9,-5) don't .work. If you plotted those points, that wouldn't pass the vertical line test. 3. has the same problem (3,4) and (3,8) and 4 with (5,9) and (5,-7).
