The mission does not gives any kind of intro or hint, no worries. We do always have the usual steps to start from. First thing to hacking is HTML the source code of the web page, it gives a lot vital information that will make an hack easy.
You can open the source code by pressing ctrl+U in firefox. Lets check out the source code and see what we got?
Ok. Here it is as well. Now if you see inside check we compare the value we entered i.e. document.a.c.value to a variable a. Just figure out the value of a and move on.
For those of you who are still wondering what to do. The answer is null.