Electrochemistry question that I find very difficult

[s3a]

Question:
http://img522.imageshack.us/img522/9718/questionc.jpg

Answer:
http://img230.imageshack.us/img230/5249/answerf.jpg

What exactly are the solutions implying? Is it that if the cell potential is positive, that it reacts? That sounds logical to me but I just wanted confirmation. How do you know to choose Fe2+ over Fe3+ or Cu+ over Cu2+? Also, how do you know when to keep something positive or negative when finding the cell's potential? Usually I would take the larger number and assign it a positive value given two reduction potentials and subtract the smaller number and get the cell potential however, something is different here and I can't understand what.

Any help would be greatly appreciated!
Thanks in advance!

[Russ]

When you do these questions you really, really need to look at an electrochemical series. Without one, this sort of question is MUCH MUCH harder.

A reaction will react (spontaneously) if the "forwards" reaction is above the "backwards" reaction on the electrochemical series.

With a series, you can just look at it and answer that question.