Ok, I've used that formula but don't know how to solve for n.
The left hand side of the inequality given by mathaddict, regardless of how it was derived, is exactly what you will get if you've used the formula you're using correctly and simplified correctly. So the question put to you remains: Can you solve that inequality?
(If you don't get that left hand side then you've made a mistake and will need to show all your work if additional help is to be given).