so, to be a linear relationship....the 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.