Well, It's very very easy. Just click the box that says "add it here", then type your answer on the box the box has buttons near the box. There is the bold, italic, underline, paste/edit equation , add symbols, and drop files. Actually you use the paste/edit equation and add symbols for answering math. It is usually used for those who likes to answer math problems, there are different equations:
>less or equal
>greater or equal
>simultanous linear equation
with two variables
While in add symbols, you don't need to know since you do know it already. But sometimes you cannot answer because of the advertisement that sometimes show below the question or your question. Then after you have already typed your answer, click on the button that says "Add your answer".
this is such a long explanation :D
Well, I hope this helps :)