I have used gaussian elimination to solve a system of 3 equations in x, y and z. I have the start of the answer but I'm not sure why the answer says a is not equal to 3. I can see a= -1 would cause an undefined result. Here is a link to the scan: ImageShack® - Online Photo and Video Hosting
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While getting to the solution for z there was factors of (a-3) that cancelled out.