We notice that both points have a y-coordinate of 9. Since they are the same, the equation must be y = 9 because the function can't "dip" or "rise", otherwise it would be a piecewise function and not a linear function. The equation y = 9 basically means that no matter what x is, y will always be 9.
First find the slope of the line. As we go from (-2, 9) to (15, 9), x increases by 17 but y does not change at all. Thus the line is y = 9, good for all x values.
