Could anyone explain this please...

I have plotted an arbitrary line y=0.83333*x +3. I needed a 2nd Line perpendicular to this going through the Origin (0,0). To be perpendicular that line needs a slope of -1*1/0.83333 = -1.2. That will be y = -1.2*x. I have used the formula as in the Diagram attached and get a a point of intersection. But this point is apparently wrong as it could be seen in the diagram. Any explanation?
Int.JPG