Can you see that these are (x,y) ordered pairs?
You could figure the slope between any two of these points.
Slope = rise/run = Δy/Δx = y2 - y1
x2 - x1
e.g.) 18 - 12 = 6 = -1
2 - 8 -6
slope = -1
We know the y intercept because the point (0,20) is given.
So I can write the equation that includes all these points using
y intercept form y = mx + b: y = -x +20
The rule is "take the opposite (or the negative) of your input value and then add 20".