You typically split up the middle term if your a-value (the coefficient on x^2) is not 1.
For example, I'd split up the middle term on 3x^2 + 4x – 4,
but not on x^2 + 4x - 12.
Technically you could on that second one, but you're making a lot of extra and unecessary work for yourself.
Does that make sense?
