let
A (4,8) and B (3,5)
step 1
find the slope m
m=(y2-y1)/(x2-x1)---------> m=(5-8)/(3-4)----> m=-3/-1----> m=3
step 2
with m=3 and point B (3,5) find the equation of the line
y-y1=m*(x-x1)-----> y-5=3*(x-3)----> y-5=3x-9----> 3x-y=4
the answer is
the equation in the form Ax+By=C is
3x-y=4
see the attached figure
