dx/x(y-1)+dy/y(x+2)=0
d/(y-1)+d/(x+2)=0
d(y-1)+d(x+2)=0
dy-d+dx+2d=0
dy+dx-3d=0
d(y+x+3)=0
d=0 ili y+x+3=0 y+x=-3
