a linear function is when the y value increases the same amount every time the x value increases by 1(or any constant interval). in this problem when x increases 1, they y value increases by 2 for the first 3 sets of numbers, but in the 4th set of numbers the x value increases by 1 and the y value increases by 2. This does not follow the same pattern and therefore makes this nonlinear. A quicker way to figure this out when you have a graph like this is to draw a line through all the points in order of increasing x values or vice versa if the line you drew is straight all the way through then it is linear and if it isn't then it is nonlinear. I hope this helped.
