Memo1.Lines.Strings[3]:=Copy(Memo1.Lines.Strings[3],1,5)+'TXT'+Copy(Memo1.Lines.Strings[3],6,Length(Memo1.Lines.Strings[3]));

2 shl (x-1)

s:=Memo1.Lines.Strings[3]
s:=Copy(s,1,5)+'TXT'+Copy(s,6,Length(s));
Memo1.Lines.Strings[3]:=s;

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, TeEngine, Series, ExtCtrls, TeeProcs, Chart;

type
  TForm1 = class(TForm)
    Chart1: TChart;
    Series1: TLineSeries;
    ScrollBar1: TScrollBar;
    procedure ScrollBar1Change(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    old:longint;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.ScrollBar1Change(Sender: TObject);
begin
     Chart1.BottomAxis.Scroll(ScrollBar1.Position-old,true)
     old:=ScrollBar1.Position;
end;

procedure TForm1.FormCreate(Sender: TObject);
var f:array [0..900] of double;
    i:longint;
begin
     for i:=0 to 900 do
     begin
          f[i]:=random(3000)*0.89;
     end;
     Chart1.Series[0].AddArray(f);
     old:=0;
end;

end.