A linear function increases at a constant rate of change, which means that for every 1 unit that x changes, y always changes at a same rate.
For example, if x moves 1, y moves 5 constantly. Every time x moves 1, y moves 5.
In this case every time x in the first table increases by 1, y increases by 6, so that is a linear function.
The second table is not linear because when x increases by 1, y decreases by 1. But the second time, when x increases by 1, y decreases by 2! -2 is not equal to -1.
The third and fourth tables are not linear for the same reasons.
