procedure TForm1.Button1Click(Sender: TObject);
var a,b,c : string;
a1,b1,c1,d,x1,x2 : integer;
begin
a:=Edit1.Text;
b:=Edit2.Text;
c:=Edit3.Text;

a1:= strToint (a);
b1:= strToint (b);
c1:= strToint (c);

d:= sqr(b)-4*a*c
x1:= (-b+sqrt(d))/2*a \\ тут строка 49
x2:= (-b-sqrt(d))/2*a \\ а тут 50

Label4.Caption:= intTostr (d);
Label5.Caption:= intTostr (x1);
Label6.Caption:= intTostr (x2);
end;

Build
   [Error] Kvadrat.pas(49): Incompatible types: 'Integer' and 'Extended'
   [Error] Kvadrat.pas(50): Incompatible types: 'Integer' and 'Extended'
   [Fatal Error] Kvadrat2.dpr(5): Could not compile used unit 'Kvadrat.pas'