How to extract Sigmoidal Membership Function's parameters giving your two sets of data?

I have x (N by 1) and y (N by 1) as given data, and I want to fit them into Sigmoidal membership function.

However, I couldn't follow the instruction here: https://www.mathworks.com/help/fuzzy/sigmf.html#mw_6eb1c0af-0e3b-4a89-bc14-234f6147bd1e

Since with x availble, it presumes the parameter a and c, and then get the y values; but for me on the other hand, I want to estimate a and c with the given x and y.

So how could I code that?

Thank you so much!