Welcome to Physics Forums!
I moved your post from the "Introducing the PF Library" thread where it was not appropriate, and put it in a new thread.
You already know how to post a response to an existing thread. To start a new thread, click the "New Topic" button at the top or bottom of a forum's thread list.
Homework help is near the top of the list of forums, as "Homework & Coursework Questions". Click on it and you'll find several sub-forums for Introductory Physics, Advanced Physics, etc. Please follow the rules that are posted there, when asking questions.