The slope-intercept form is y = mx + b
From two points we can easily determine the slope, m = (y2-y1)/(x2-x1)
m = (-9-(-9))/(-3-1) = 0/-4 = 0
Since the slope is 0 we know it's a line parallel to the x-axis.
Using the slope-intercept form, y = 0(x) + b ⇒ y = b
But we don't know what the y-intercept (b) is yet. Using the point-slope form we can determine b
Point-slope: (y - y1) = m(x - x1)
(y - (-9)) = 0(x - (-3))
y + 9 = 0
y = -9
So the equation is y = -9.