The relationship is linear.
The reason why is because each time x increases by 1, the value of y increases by 3. In other words, the slope is 3 and it is constant no matter what two points you pick
x = input
y = output
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Extra Info:
slope = rise/run
slope = (change in y)/(change in x)
slope = 3/1
slope = 3
The y intercept is (0,6). Think backwards in terms of the pattern going on.
Or you can plug m = 3 and (x,y) = (1,9) into y = mx+b and solve for b to get b = 6