Trouble with Challenge Task 3 of 3 from Adding Settings to a Wordpress Widget

<?php

    class My_Plugin_Widget extends WP_Widget {

        function my_plugin_widget() {
            parent::__construct( false, 'My Plugin Widget' );
        }

        function widget( $args, $instance ) {
            extract( $args );
            $title = apply_filters( 'widget_title' , $instance['title'] );
      $num_courses = $instance['num_courses'];


        }

        function update( $new_instance, $old_instance ) {
            $instance = $old_instance;
            $instance['title'] = strip_tags($new_instance['title']);
      $instance['num_courses'] = strip_tags($new_instance['num_courses']);

            return $instance;
        }

        function form( $instance ) {
            $title = esc_attr($instance['title']);
      $num_courses = esc_attr($instance['num_courses']);

            require( 'inc/widget-fields.php' );
      require( 'inc/widget-front-end.php' );
        }

    }

    function my_plugin_register_widgets() {
        register_widget( 'My_Plugin_Widget' );
    }

    add_action( 'widgets_init', 'my_plugin_register_widgets' );

?>