Some extra info:
If you look at the discriminant, , of a quadratic expression , you can work out:
1. How many solutions it has
2. Whether or not it can be factored
If , there are 2 real solutions.
If , there is 1 real solution.
If , there are no real solutions.
If is a perfect square, that is, if , then the expression can be factored, else it can't be.
For your original incorrect expression , , and since 28 isn't a perfect square it can't be factorised.
Whereas has discriminant , which is a perfect square so it can.
There are other methods of finding roots, the first is completing the square, and the second is the quadratic formula:
The quadratic formula is very useful and you probably should remember it. For any expression , the solutions are