Yes there is an error in the student's work. The error is that he is considering only single value of x. To correct the error he/she must calculate all the possible values of x. The process is shown in the paper.
Https://answers.yahoo.com/question/index?qid=20120624121412AAV4Cjw Hope that helps! If the link doesn't work if you click on it, then just copy and paste.
