typedef struct Point
{
    unsigned int x;
    unsigned int y;
} POINT;

typedef struct Matrix
{
    long An;
    long Bn;
    long Cn;
    long Dn;
    long En;
    long Fn;
    long Divider;
} MATRIX;

lcdPtr is the pointer to POINT variable with this content:    POINT displaySample[3] = {{80,48},{400,432},{720,240}}
touchPtr is the pointer to POINT variable with the touch screen analog readings
matrixPtr is the pointer to the matrix, where the final calculated matrix will be saved

int calibrationMatrix( POINT *lcdPtr, POINT *touchPtr, MATRIX *matrixPtr)
{

    int  retValue = 1 ;

    matrixPtr->Divider = ((touchPtr[0].x - touchPtr[2].x) * (touchPtr[1].y - touchPtr[2].y)) -
                         ((touchPtr[1].x - touchPtr[2].x) * (touchPtr[0].y - touchPtr[2].y)) ;

    if( matrixPtr->Divider == 0 )
    {
        retValue = 0 ;
    }
    else
    {
        matrixPtr->An = ((lcdPtr[0].x - lcdPtr[2].x) * (touchPtr[1].y - touchPtr[2].y)) -
                        ((lcdPtr[1].x - lcdPtr[2].x) * (touchPtr[0].y - touchPtr[2].y)) ;

        matrixPtr->Bn = ((touchPtr[0].x - touchPtr[2].x) * (lcdPtr[1].x - lcdPtr[2].x)) -
                        ((lcdPtr[0].x - lcdPtr[2].x) * (touchPtr[1].x - touchPtr[2].x)) ;

        matrixPtr->Cn = ((touchPtr[2].x * lcdPtr[1].x) - (touchPtr[1].x * lcdPtr[2].x)) * touchPtr[0].y +
                        ((touchPtr[0].x * lcdPtr[2].x) - (touchPtr[2].x * lcdPtr[0].x)) * touchPtr[1].y +
                        ((touchPtr[1].x * lcdPtr[0].x) - (touchPtr[0].x * lcdPtr[1].x)) * touchPtr[2].y ;

        matrixPtr->Dn = ((lcdPtr[0].y - lcdPtr[2].y) * (touchPtr[1].y - touchPtr[2].y)) -
                        ((lcdPtr[1].y - lcdPtr[2].y) * (touchPtr[0].y - touchPtr[2].y)) ;

        matrixPtr->En = ((touchPtr[0].x - touchPtr[2].x) * (lcdPtr[1].y - lcdPtr[2].y)) -
                        ((lcdPtr[0].y - lcdPtr[2].y) * (touchPtr[1].x - touchPtr[2].x)) ;

        matrixPtr->Fn = ((touchPtr[2].x * lcdPtr[1].y) - (touchPtr[1].x * lcdPtr[2].y)) * touchPtr[0].y +
                        ((touchPtr[0].x * lcdPtr[2].y) - (touchPtr[2].x * lcdPtr[0].y)) * touchPtr[1].y +
                        ((touchPtr[1].x * lcdPtr[0].y) - (touchPtr[0].x * lcdPtr[1].y)) * touchPtr[2].y ;
    }

    return( retValue ) ;

}

When I want to calculate the screen pos from an analog reading I use this code:
    long xAnalogReading, yAnalogReading;
    x = ((matrix.An * xAnalogReading) + (matrix.Bn * yAnalogReading) + matrix.Cn) / matrix.Divider;
    y = ((matrix.Dn * xAnalogReading) + (matrix.En * yAnalogReading) + matrix.Fn) / matrix.Divider;