To be honest, I have no idea what you are trying to say (or do) here.
There are issues(*) in deciding the correct
angle if all you know is the tangent (or the ratio (y-a)/(x-b)). But in your problem you already know the angle
−3π/4 and you're trying to get to the ratio (x-a)/(x-b). In which case you can just take the tangent, and that's fine. You should never be worrying about "adding 180 etc."
(*) The issue comes from the fact that if you want to find arg(x+iy) by simply looking at (x/y), then you obviously can't tell the difference between (x+iy) and -(x+iy) (since the second gives you the ratio (-x/-y) which is the same as (x/y)). So if you want the argument, you need to be careful.
To be honest, I think it would be helpful if you posted your original question
in full. There are subtleties depending on exactly how you are supposed to be dealing with the fact that
arg(z−w)=θ only defines a half-line, whereas something of the form
x−ay−b=tanθ defines a full line (possibly excluding the point (a, b)).