Please use a word processing program to type up your solutions. Try to write your solutions clearly and succinctly. Use complete sentences. Poorly written solutions correct solutions will not recieve full credit. Here is some advice on mathematical writing, from a course at Purdue (thanks to Prof. Rogaway for pointing this out!).
But don't get so hung up on writing a perfect solution or getting the correct result that you instead hand in nothing! There will be partial credit!
While you are welcome to use books, the Web, or your peers for help with the homework, your solutions must be written up clearly in your own words. This means you should be alone, and not looking at any online or hardcopy reference materials.