Your code fixed is below:
program hinhvuong;
uses crt;
var a,c,s:real;
begin
clrscr;
write('nhap so canh:');readln(a);
writeln('chu vi hinh vuong');
c:=a+a+a+a;
writeln('ket qua:', c);
writeln('dien tich hinh vuong');
s:=a*a;
writeln('ket qua:', s);
readln;
end.