Each value in the 'x' column (on the left) is unique. There are no repeated x values. This means we do have a function.
Put another way, each x value leads to exactly one y value. This is another way to see that we have a function. A function input must have one output or else it would lead to confusion or ambiguity. For example, let's say you wanted to convert from celsius to fahrenheit. You would use the conversion function F = (9/5)C+32. The idea is whatever celsius C value you plug in, you should get ONE and only one fahrenheit value. Otherwise, it would be impossible to know which fahrenheit temperature to go with.
Yet another way is to graph each (x,y) point and notice it is impossible to draw a single vertical line through more than one point mentioned in the table. This graph passes the vertical line test to further confirm that we do have a function.
