As x increases from 9 to 12 (a change of 3), y increases from -2 to 0 (a change of 2). Thus, the slope of this line is 2/3.
Subbing this info and the coordinates of one of the points into y = mx + b, we get:
-2 = (2/3)(9) + b. Solving for b: b = -2 -18/3 = -8. Then the equation of this line is:
y = (2/3)x - 8.
Check by subst.: Does (12,0) satisfy this equation? 0 = (2/3)(12) - 8? Yes.
