• To make it compatible with the drawpoly function, I converted the 2D matrices holding the coordinates of the points to a 1D matrix which is acceptable by the graphics function. 
  
  
• Next time, use line function. The coding becomes simpler, and easier to understand.

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#include<graphics.h>
#include<iostream.h>
#include<conio.h>
 
int n=4;     //Variables for drawpoly() function
int p[8];
int d[8];
 
int Sx,Sy;   //Scaling factors.
 
 
 
void Input()
{
cout<<"\n Enter the scaling factor of x:";
cin>>Sx;
cout<<" Enter the scaling factor of y:";
cin>>Sy;
 
cout<<"\n Using default triangle values \n  
                            | A(0,-20), B(20,-20), C(20, 20) |";
cout<<"\n\n  Click to Continue... ";
getch();
cleardevice();
 
p[0]=0;
p[1]=-30;
p[2]=-30;
p[3]=+30;
p[4]=+30;
p[5]=+30;
p[6]=0;
p[7]=-30;
 
}
 
 
void Draw(int Xmid, int Ymid)
{
 
for(int i=0; i<8; i++)
{
        if( i%2==0 )
        d[i]=Xmid+p[i];
        else
        d[i]=Ymid+p[i];
}
 
drawpoly(n,d);
getch();
}
.
void Scale()
{
int i,j,k;
 
// Matrix with Triangle Coordinates
int xy1[3][3] = { p[0], p[1], 1,
                  p[2], p[3], 1,
                  p[4], p[5], 1,
                };
 
// Matrix with Scaling Factors
int SI[3][3] = { Sx, 0, 0,
                 0, Sy, 0,
                 0,  0, 1
               };
 
// Matrix with Coordinates after Scaling.
int XY1[3][3];
 
//Matrix multiplication: xy1 x SI = XY1
for(i=0; i<3; i++)
{
for(j=0; j<3; j++)
{
        XY1[i][j]=0;
for(k=0; k<3; k++)
{
        XY1[i][j]+=xy1[i][k]*SI[k][j];
} } }
 
// Converting Final Matrix into Single Array
k=0;
for(i=0; i<3; i++)
{
for(j=0; j<2; j++)   // Till 2 because '1' not needed.
{
p[k++]=XY1[i][j];
} 
}
p[k++]=XY1[0][0];
p[k]=XY1[0][1];
}
 
void main()
{
int gdriver=DETECT,gmode;
initgraph(&gdriver,&gmode,"C:/TC/BGI");
Input();
 
setcolor(CYAN);
outtextxy(10,getmaxy()*0.25," BEFORE SCALING: ");
Draw(getmaxx()*0.25,getmaxy()*0.5 );
 
Scale();
 
setcolor(GREEN);
outtextxy(getmaxx()*0.5,getmaxy()*0.25," AFTER SCALING: ");
Draw(getmaxx()*0.75, getmaxy()*0.5);
 
closegraph();
}