Question 1013117
When the slope is -2, the line has the form y=-2x
the y-intercept gives you the last term.  
You have two ways to get it.
One is to use y=-2x+b
When y=0, x=4.  Substitute them in
0=-2(4)+b
0=-8+b
b=8
y=-2x+8
===============
The other way is to use the point slope formula.
y-y1=m(x-x1), where (x1,y1) are coordinates of a point.
y-0=-2(x-4)
y=-2x+8
==============
Either one works.