procedure TfmMain.Button1Click(Sender: TObject);
var
  Pt1,Pt2:TPoint;
  dx,dy:integer;
  lx,ly,kx,ky:double;
  LineWidth:integer;
  ArrowLength:integer;
  ArrowWidth:integer;
  Coor:array[ 1 .. 3 ] of TPoint;
begin
  Pt1.X:= 210 ;
  Pt1.Y:= 300 ;
  Pt2.X:= 50 ;
  Pt2.Y:= 200 ;
  LineWidth:= 3 ;
  ArrowLength:= 15 ;
  ArrowWidth:= 5 ;

  dx:=Pt2.X-Pt1.X;
  dy:=Pt2.Y-Pt1.Y;
  kx:=Sign(dx);
  ky:=Sign(dy);
  if Abs(dx)>Abs(dy) then
    ky:=kx*dy/dx
  else
    kx:=ky*dx/dy;
  lx:=Pt2.X;
  ly:=Pt2.Y;
  Coor[ 1 ].x:=Round(lx-ArrowLength*kx+ArrowWidth*ky);
  Coor[ 1 ].y:=Round(ly-ArrowLength*ky-ArrowWidth*kx);
  Coor[ 2 ].x:=Round(lx);
  Coor[ 2 ].y:=Round(ly);
  Coor[ 3 ].x:=Round(lx-ArrowLength*kx-ArrowWidth*ky);
  Coor[ 3 ].y:=Round(ly-ArrowLength*ky+ArrowWidth*kx);

  Canvas.Pen.Width:=LineWidth;
  Canvas.Pen.Color:=clRed;
  Canvas.MoveTo(Pt1.X,Pt1.Y);
  Canvas.LineTo(Pt2.X,Pt2.Y);
  Canvas.MoveTo(Coor[ 1 ].X,Coor[ 1 ].Y);
  Canvas.LineTo(Coor[ 2 ].X,Coor[ 2 ].Y);
  Canvas.LineTo(Coor[ 3 ].X,Coor[ 3 ].Y);
end;