Z-buffer Algorithm problem in finding depth of surface of polygon.

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C#

graphics

I am trying to use z-buffer algorithm to find the visible surface detection for my college computer graphics project.I have used following co-ordinate system that the positive x-axis to the right, the positive z-axis upward, and the positive y-axis forward on the screen. My maximum depth is 200 units.
Problem is that when I calculate the depth of all surface of cube I found the depth of back and front accurate but depth of other side is more than my maximum depth.
My code for calculating co-efficient of plane is

public Coefficient_Of_Plane Coefficient_Value(_3Dpoint pt1, _3Dpoint pt2, _3Dpoint pt3)
       {
          // double Y = 0;
           Coefficient_Of_Plane pt;
           pt = new Coefficient_Of_Plane();
           pt.A = (pt2.z - pt3.z) * (pt1.y - pt2.y) - (pt1.z - pt2.z) * (pt2.y - pt3.y);
           pt.B = (pt2.x - pt3.x) * (pt1.z - pt2.z) - (pt1.x - pt2.x) * (pt2.z - pt3.z);
           pt.C = (pt2.y - pt3.y) * (pt1.x - pt2.x) - (pt1.y - pt2.y) * (pt2.x - pt3.x);
           pt.D = - pt1.x * (pt2.y * pt3.z - pt2.z * pt3.y) + pt1.y * (pt2.x * pt3.z - pt2.z * pt3.x) - pt1.z * (pt2.x * pt3.y - pt2.y * pt3.x);
           return pt;

       }

and finding the depth value is

public double DepthValue(Coefficient_Of_Plane surface,int x,int y)
      {


          double z = 0;
          if (surface.B != 0)
              z = (-surface.A * x - surface.C * y - surface.D) / surface.B;
          return z;
      }

I need a help to solve this problem and I am also ready to post other part of code if required and further description of problem if my question is incomplete.