ax+by=c is form
start from y=mx+b form
we know that b=y intercept or b in (0,b) when x=0
so
we got (0,-4)
so b=-4
y=mx-4
find m
(5,1)
x=5 and y=1 is true
1=5m-4
add 4 both sides
5=5m
divide 5
1=m
slope is 1
y=1x-4
y=x-4
convert
minus x both sides
-x+y=-4
times -1
x-y=4
