Started a new online course to try and get better at math, but the course has a few dead links and I'm missing some information. I have this question:

"For what values are the following undefined"

x^2 - 2x + 3

___________

x^2+2x+1

first I expand

x^2- 2x + 3

__________

(x+1)(x+1) [or x+1^2]

Then I don't know what to do.

I've looked at the easier examples they give me and when they give an answer for an "undefined value" it is the x-intercepts/zeros of the function. Can someone explain to me what exactly they mean by "undefined values"