Sql in Model - Undefined variable: db

This is my symple model:

<?php namespace App\Models;

use CodeIgniter\Model;


class banner extends Model {

$query_ = "SELECT img_banner FROM tab_4 WHERE m = 12";

$query = $db->query($query_);

return $query->getResultArray();

}

?>

I receive this error: Undefined variable: db

Which is the correct way for resolve the issue?