Assuming you want the y-intercept of the line through those two points, you can find it a couple of different ways.
The slope of the line is ...
... slope = (change in y)/(change in x) = (6-18)/(6-2) = -12/4 = -3
We are given the y-value for x=2. When x decreases by 2 to x=0, then the y-value changes by (-2)·(-3) = +6, from 18 to 24.
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You can also write the equation of the line in point-slope form as ...
... y = -3(x -2) +18
Again, when x=0, the y-intercept is ...
... y = -3(0 -2) +18 = 6 +18 = 24
The y-intercept of the line through (2, 18) and (6, 6) is 24.
