Let me just go through that working again:

or

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

For 1.

If , there are 2 real solutions.

If , there is 1 real solution.

If , there are no real solutions.

For 2.

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