Error in simple-qa.php line 64 after updating to PHP 8.x

$args = array(
    'label' => __('Q&A', 'simple-qa'),
    'labels' => $labels,
    'public' => false,
    'can_export' => true,
    'show_ui' => true,
    'menu_position'     => 21+$x-1,
    '_builtin' => false,
    'capability_type' => 'post',
    'menu_icon'         => 'dashicons-format-chat',
    'hierarchical' => false,
    'rewrite' => array( "slug" => "qa".$x."" ),
    'supports'=> array('title', 'editor', 'comments'),
    'show_in_nav_menus' => true
  );

Looks like “21+$x-1” is bad format in PHP8.x any idea how to fix this?