i want to know the problem in the fourth task of this challenge

public class ForumPost {
  private User mAuthor;
  private String mTitle;
  private String mDescription;
public ForumPost(User nAuthor,String title,String description){
  nAuthor=mAuthor;
    mTitle = title;
    description= mDescription;
  }
  public String getDescription(){
  return mDescription;
  }


  public User getAuthor() {
    return mAuthor;
  }


  public String getTitle() {
    return mTitle;
  }

  // TODO: We need to expose the description
}

public class User {
  private String mFirstName;
   private String mLastName;

  public User(String firstName, String lastName) {
    // TODO:  Set the private fields here    

 mFirstName = firstName;
    mLastName = lastName;
  }

  public String getFirstName(){
return mFirstName;
  }
  public String getLastName(){
  return mLastName;
  }
}

public class Forum {
  private String mTopic;

  public String getTopic() {
      return mTopic;
  }
    public Forum(String ltopic){
mTopic = ltopic;
  }


  public void addPost(ForumPost post) {
      System.out.printf("New post from %s %s about %s.\n",
                         post.getAuthor().getFirstName(),
                         post.getAuthor().getLastName(),
                         post.getTitle());

  }
}

public class Example {

  public static void main(String[] args) {
    System.out.println("Starting forum example...");
    if (args.length < 2) {
       System.out.println("first and last name are required. eg:  java Example Craig Dennis");
    }
     Forum forum = new Forum("Java");
    // Take the first two elements passed args
    User author = new User("Neelesh","Tewani");
    // Add the author, title and description
    ForumPost post = new ForumPost(author.getFirstName(),author.getLastName(),"challenge completed","yes i have done the challenge");
    forum.addPost(post);
  }
}