DELPHI - How to make rotation of shape around point

Suppose this easy class for shape - triangle:

TShape = class( TObject )
  p1 : TPoint;
  p2 : TPoint;
  p3 : TPoint;

  procedure Moving( _iDeltaX, _iDeltaY : integer );
  procedure Rotation( _iAngle : integer );
  procedure RotationAroundXY( _x, _y : integer; _iAngle : integer );
end;

And initialization:

pShape := TShape.Create;
pShape.p1.X := 300;  pShape.p1.Y := 250;
pShape.p2.X := 250;  pShape.p2.Y := 280;
pShape.p3.X := 350;  pShape.p3.Y := 330;

For rotation shape around point use this code:

procedure TShape.RotationAroundXY( _x, _y : integer; _iAngle : integer );
var
  iNewX, iNewY : integer;
  dCos, dSin : double;
  dRadian : double;
  p1x, p1y, p2x, p2y, p3x, p3y : integer;
begin
  { -- angle }

  dRadian := ( 2 * pi ) / ( 360 / _iAngle );
                     
  dCos := cos( dRadian );
  dSin := sin( dRadian );

  { -- point for rotation around (in other case the rotation is around start of coords) }

  p1x := p1.x - _x;
  p1y := p1.y - _y;
  p2x := p2.x - _x;
  p2y := p2.y - _y;
  p3x := p3.x - _x;
  p3y := p3.y - _y;

  { -- calculation of the positions }

  iNewX := round( ( p1x * dCos ) - ( p1y * dSin ) );
  iNewY := round( ( p1y * dCos ) + ( p1x * dSin ) );
  p1.X := iNewX + _x;
  p1.Y := iNewY + _y;

  iNewX := round( ( p2x * dCos ) - ( p2y * dSin ) );
  iNewY := round( ( p2y * dCos ) + ( p2x * dSin ) );
  p2.X := iNewX + _x;
  p2.Y := iNewY + _y;

  iNewX := round( ( p3x * dCos ) - ( p3y * dSin ) );
  iNewY := round( ( p3y * dCos ) + ( p3x * dSin ) );
  p3.X := iNewX + _x;
  p3.Y := iNewY + _y;
end;

Output: