back := '';
But you can check for palindrome in another way:
len := length(x);
is_palindrome := true;
for x := 1 to len do begin
if s[x] <> s[len - x + 1]
then is_palindrome := false; break; end;
end;
if is_palindrome
then begin
writeln('It''s a palindrome');
end
else begin
writeln('It''s not a palindrome');
end;