how to send email with a different subject in laravel

here I want to send an email with a different subject, here I have created a SendEmail class, then I want to create a subject but I get an error, Call to undefined the Illuminate \ Mail \ PendingMail :: subject () method this my SendMail code

  public function build()
    {
        // return $this->view('view.name');
          return $this->from('admin@surveynesia.id')
                   ->view('sendmail');             
    }

and this my controller for send mail

Mail::to($auth->email)
          ->subject('re-aktivasi'.$user->name)
          ->send(new SendEmail($user));

via Chebli Mohamed