Before you can complete the square, you need the coefficient on the n^{2} term to be 1. In other words, you need to divide all of you terms by 2.
After dividing by 2, you will have n^{2}  10n and whatever other information is in the problem.
To find the number that needs to be added to each side, divide the coefficient on the n term by 2 and then square it. In other words:
(10/2)^{2} = 5^{2} = 25
After adding 25 to each side, you can then make the left side into a perfect square:
(n  5)^{2}
Notice that the last term is the square root of what you added to each side.
2/5/2013

John R.