First, find m, the slope of the line.
(1-3)/(5-1) =
-2/4 =
-1/2
Next, choose either point to write the point slope form. I will show the outcome of both points.
1. Using point (1, 3)
y-y1=m(x-x1)
y-3=(-1/2)(x-1)
————————
y-3=-1/2x+1/2
y=-1/2x+7/2
2. Using point (5,1)
y-y1=m(x-x1)
y-1=-1/2(x-5)
————————
y-1=-1/2x+5/2
y=-1/2x+7/2
The outcome is the same.