How to Hide the "Qwizcards" Admin Menu

function plt_hide_qwiz_online_quizzes_and_flashcards_menus() {
	//Hide the "Settings → Qwizcards" menu.
	remove_submenu_page('options-general.php', 'qwiz-admin');
}

add_action('admin_menu', 'plt_hide_qwiz_online_quizzes_and_flashcards_menus', 11);

Where do I put this code?