Help with inflection point in a curvilinear relationship

Hi all,

I am reading an article that does a panel poission regression and finds out the following function:

Y= 2,22 + 0,08X1 -0,0001 X1SQ +1,42* X2 -4,17* X2SQ+1,24 X3

both X1 and X2 have quadratic terms (I put SQ). then the article goes on to indicate the inflection point (on y axis) for variable X1 and then X2.

Could anyone please help me how to find the inflection point bwith poission regression? I understand that one differentiates the equation based on X1 (or X2) and then equates to zero; takes the value that leads to zero and then plugs into the equation above to find the inflection value on Y...But I cant get the numbers the authors write. What am I missing?
Thank you very much
PS: I will be more than happy to email the article


TS Contributor
Have you tried contacting the authors? I'm sure they'd be thrilled to hear from someone who actually read their article and wants to discuss it....