You've got the points:
(1,1) (2,-2) (3,-5) (4,-8)
First find the slope.
-2-1 / 2-1 = -3/1 = -3
-5- - 2 / 3-2 = -3/1 = -3
-8 - -5/ 4-3 = -3/1 = -3
m = -3 = slope
Now plug it into point slope intercept form
y-y₁=m(x-x₁)
y - 1 = -3(x - 1)
y - 1 = -3x +3
y = -3x + 4
