Each time x goes up by 1, y also goes up by 1 as well.
slope = rise/run = 1/1 = 1
Or we can use the slope formula to get
m = (y2-y1)/(x2-x1) = (3-2)/(2-1) = 1/1 = 1
I used the first two rows as the points to plug into the slope formula.
You can pick any two rows you want and you'll get the same slope value.
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Because we get the same result of 1 each time, this means the rate of change is constant and this function is linear. Plotting all four points will show a single straight line goes through all of them. See diagram below.