This is probably the most simple problem you can have on Khan Academy. Simply scroll down to the bottom part of the screen under the post to the end of the white. If there is a text box, then great! If not, there is a simple one click answer. Just click "sign in" (It will say "SIGN IN to comment) and scroll back down to the bottom of the page where a text box will appear. i will post a picture as a comment to this post later. Stay tuned!