Linear from a table

A straight line climbs by the same amount every time. So look at the steps between the numbers.

First check the x row is evenly spaced: 3, 3, 3. Every step is 3, so the readings are evenly spaced.

Now take the steps in y: 18, 54, 90.

The steps are not all the same, so the points bend instead of climbing steadily: x goes up by 3 every time, but the steps in y are 18, 54, 90 — they change, so the points do not lie on a line.