Explain why the relationship is not linear.

So, to be a linear relationshipthe difference between the two consecutive "x" values and the difference between the two consecutive "y" values must be same.

In this case,
if we look at the x values , 4 - 3 = 1
                                          3 - 2 = 1
                                          2 - 1 = 1
they are all the same,
BUT, if we look at the y values, 11.5 - 8.5 = 3
                                                   8.5 - 6.5 = 2
                                                   6.5 - 4.5 = 2
Here, the differences between the "y" values are all not the same.

SO, the relationship is not linear.