Query for a table value in ajax Class format poo with extends

I have this problem and I want to consult directly, I get an error, if someone knew how to do it better, I would like to be instructed.

$user = $db->facultad->getAll("SELECT cuenta, contraseña FROM cuentas WHERE cuenta='MAT2018A001'");         
if (!empty($user)){
   $password = $user[0]['contraseña'];
   echo $password."<br>";
}