You have made a few illogical attempts. The only things you need to try are the factors of .
You will soon notice, having tried these, that neither works: indeed, it doesn't factor. If you've struggled with the formula, why not give completing the square a jab?
So you want something of the form:
where k a number you need to work out.
Edit: Please doublecheck your factors: there's a sign error in your calculations. This quadratic doesn't factor.