I've encountered the following question:

Find the coordinates of the points on the curve , where the tangent is horizontal.

My plan was to take the derivative and then try to find where x goes to infinity by taking the limit. Things quickly fell apart.

I managed to get the derivative of however when I check my work with WolframAlpha I'm told I'm wrong with . Now I can see the identity. However I see -1 and WolframAlpha is saying +1. My train of thought is . So where is my basic algebra failing me with +1?

Furthermore, is my thought process correct by thinking I need to find the limit as ?