Joining two tables with the same rows

Hi! Can someone help me here? I just got this code from other topics and use it for my system but i have a problem

$idList = join (',', array_map('intval', array_keys($_POST['answers'])));		
$sql = "SELECT QuestionId, answer
FROM tblquestion_mc WHERE QuestionId IN ($idList";

what i am trying to do is to join my table: tblquestion_default to that code above but i don't know how to. 

My tables contain this:

table name: tblquestion_mc

QuestionId | Answer

table name" tblquestion_default

QuestionId | Answer