I think you're talking about exact equations. See Chris's tutorial, post # 2 (scroll down a little bit).
xy' + y - 2x = 0
We need to learn the method where you re-arrange too
N(x,y)dy + M(x,y)dx = 0
Then solve somehow getting d(y-2x)/dy = 1 etc...
Sorry I do not know the name of this method, but I really could do with a step by step sort of guide on how to do it for any example, as I can solve this in many other ways, but don't understand this particular method of solving 1st Order ODE's