Point[] points = new Point[3] { left, top, right };

DrawField();DrawCoordinateSystem();

graph.FillPolygon(Brushes.ForestGreen, points);

Point[] pointsCopy = (Point[]) points.Clone();

x = A.x + (A.x - P0.x)
y = A.y + (A.y - P0.y)

|B*P0.x + C*P0.y,  C|
      |C*P1.x - B*P1.y, -B|
 A.x = --------------------- ,
          -B*B - C*C

      |B,  B*P0.x + C*P0.y|
      |C,  C*P1.y - B*P1.y|
 A.y = ---------------------
          -B*B - C*C

 B = P2.x - P1.x
 C = P2.y - P1.y

A

Point A = new Point( -width, -(a / b) * -width - c / b );
Point B = new Point( width, -(a / b) * width - c / b );

float[,] L = new float[2, 3] {
    { A.X, A.Y, 1 },
    { B.X, B.Y, 1 }
};

float[,] X = new float[3, 3] {
    { points[0].X, points[0].Y, 1 },
    { points[1].X, points[1].Y, 1 },
    { points[2].X, points[2].Y, 1 }
};

private static float[,] MultiplyMatrix(float[,] a, float[,] b)
{
    float[,] product = new float[a.GetLength(0), b.GetLength(1)];

    for (int row = 0; row  product.GetLength(0); row++)
        for (int col = 0; col  product.GetLength(1); col++)
            // Multiply the row of A by the column of B
            for (int inner = 0; inner  a.GetLength(1); inner++)
                product[row, col] += a[row, inner] * b[inner, col];

    return product;
}

float ys = -(a / b) * 0 - c / b;

float[,] T = new float[3, 3] {
    { 1, 0, 0 },
    { 0, 1, 0 },
    { 0, -ys, 1 }
};

L = MultiplyMatrix(L, T);
X = MultiplyMatrix(X, T);

double radians = Math.Atan(-(a / b));

float[,] R = new float[3, 3] {
    { Math.Cos(radians), -Math.Sin(radians), 0 },
    { Math.Sin(radians), Math.Cos(radians), 0 },
    { 0, 0, 1 }
};

L = MultiplyMatrix(L, R);
X = MultiplyMatrix(X, R);

float[,] Rr = new float[3, 3] {
    { 1, 0, 0 },
    { 0, -1, 0 },
    { 0, 0, 1 }
};

X = MultiplyMatrix(X, Rr);

float[,] Rh = new float[3, 3] {
    { Math.Cos(radians), Math.Sin(radians), 0 },
    { -Math.Sin(radians), Math.Cos(radians), 0 },
    { 0, 0, 1 }
};

L = MultiplyMatrix(L, Rh);
X = MultiplyMatrix(X, Rh);

float[,] Th = new float[3, 3] {
    { 1, 0, 0 },
    { 0, 1, 0 },
    { 0, ys, 1 }
};

L = MultiplyMatrix(L, Th);
X = MultiplyMatrix(X, Th);

for (int i = 0; i  p.Length; i++)
{
    points[i].X = X[i, 0];
    points[i].Y = X[i, 1];
}

graph.FillPolygon(Brushes.RoyalBlue, points);