I am posting solution for challenge in module " Use Simple Variables and Formulas ".

Challenge :-  

Create a Visualforce page that shows user information

Create a Visualforce page that displays the first name of the currently logged-in user.

Challenge Requirements

• Page name: DisplayUserInfo
• The displayed user information must be generated dynamically from the logged-in user

Solution :- 

<apex:page >

  <p>

      {! $User.FirstName}

    </p>  

    <p>

        {! $User.LastName }

    </p>

    {! $User.Username }

</apex:page>