You can simply plot these points on a graph and see where the line goes. It goes through the point (5, 20), so n = 5.
You can recognize the difference between the given x-values is 3 and the difference between the given y-values is 15. Thus adding 30=2×15 to the last y-value will correspond to adding 6=2×3 to the last x-value: -1 +6 = 5.
