How to display node add answer form on a question page using the Answers module?
The most recent version of the Answers module (= 7.x-4.0-rc2) already comes, out-of-the-box, with a "answer node add form on the question page" (as in your question). So you don't need any other modules, such as Advanced form block (it all happens automagically).

To experience it yourself, try out a demo of it. Here are the steps to do so:


Head over to the demo site.
Just accept the defaults to create a sandbox demo site (hit the create sandbox button, and wait for the process to complete).
Login to the site (using the suggested defaults).
Create at least 1 question using either the "Add content" menu link and then selecting "Question", or just enter relative path /node/add/question. And complete the title and body of the question.
After you save the question, you'll see the question that's created (in view mode). And below the question, you should see "No answers yet." (since you just completed posting the question). And then there is the "Your Answer" form right below that. That is the form you can use to post an answer to the question.
If you then use that (answer) form to post your answer, and also save that, the (first) answer is shown below the question, and below that answer there is now the "Your Answer" form again (to allow for posting an additional answer).
Use the Questions tab (or enter relative path /questions, to get a list (a tabular display) of all existing questions so far (there should be at least one if you completed the previous step).


Note: you may also want to enable the Answers Theme (sub-module of Answers, new as of 7.x-4.x). After you do, return to the question you created before, and see how the look-and-feel of both the question and related answer(s) below it changed ... Here is a sample screenprint of it: