[daveFNbuck]

I took a quick look through the first set of exercises and a lot of the problems are open-ended and don't have specific answers. Seeing a different answer than yours doesn't tell you much about whether your answer is correct.

Even if you do have a similar proof to the one in the answer key, that doesn't mean your proof is correct. Correctness can often hinge on very subtle distinctions. You really need a TA or instructor to read your proof if you want meaningful feedback.