Linear function definition: a function that will create a straight line when graphed.
11 is not a linear function (not a straight line), but 12 is.
For the tables, you can either plot out the graph or just look at the number patterns. A linear function's numbers will always increase/decrease accordingly at a constant rate. For example, in 14, you notice that as x increases by 1, y decreases by 2. This means it is a linear graph.
In 13, as x increases by 1, the output y increases but at a different rate each time. It increased by 3 at first, then 5, then 7. Hence, it is not a linear function. What is it then? It is an exponential function.